A Short Biography
Johannes Kepler was born at 2:30 PM on December 27, 1571, in Weil der Stadt, Württemberg, in the Holy Roman Empire of German Nationality. He was a sickly child and his parents were poor. But his evident intelligence earned him a scholarship to the University of Tübingen to study for the Lutheran ministry. There he was introduced to the ideas of Copernicus and delighted in them. In 1596, while a mathematics teacher in Graz, he wrote the first outspoken defense of the Copernican system, the Mysterium Cosmographicum. Kepler's family was Lutheran and he adhered to the Augsburg Confession a defining document for Lutheranism. However, he did not adhere to the Lutheran position on the real presence and refused to sign the Formula of Concord. Because of his refusal he was excluded from the sacrament in the Lutheran church. This and his refusal to convert to Catholicism left him alienated by both the Lutherans and the Catholics. Thus he had no refuge during the Thirty-Years War.
Kepler was forced to leave his teaching post at Graz due to the counter Reformation because he was Lutheran and moved to Prague to work with the renowned Danish astronomer, Tycho Brahe. He inherited Tycho's post as Imperial Mathematician when Tycho died in 1601. Using the precise data that Tycho had collected, Kepler discovered that the orbit of Mars was an ellipse. In 1609 he published Astronomia Nova, delineating his discoveries, which are now called Kepler's first two laws of planetary motion. And what is just as important about this work, "it is the first published account wherein a scientist documents how he has coped with the multitude of imperfect data to forge a theory of surpassing accuracy" (O. Gingerich in foreword to Johannes Kepler New Astronomy translated by W. Donahue, Cambridge Univ Press, 1992), a fundamental law of nature. Today we call this the scientific method. In 1612 Lutherans were forced out of Prague, so Kepler moved on to Linz. His wife and two sons had recently died. He remarried happily, but had many personal and financial troubles. Two infant daughters died and Kepler had to return to Württemburg where he successfully defended his mother against charges of witchcraft. In 1619 he published Harmonices Mundi, in which he describes his "third law." In spite of more forced relocations, Kepler published the Epitome Astronomiae in 1621. This was his most influential work and discussed all of heliocentric astronomy in a systematic way. He then went on to produce the Rudolphine Tables that Tycho had envisioned long ago. These included calculations using logarithms, which he developed, and provided perpetual tables for calculating planetary positions for any past or future date. Kepler used the tables to predict a pair of transits by Mercury and Venus of the Sun, although he did not live to witness the events. Johannes Kepler died in Regensburg in 1630, while on a journey from his home in Sagan to collect a debt. His grave was demolished within two years because of the Thirty Years War. Frail of body, but robust in mind and spirit, Kepler was scrupulously honest to the data.
Showing posts with label Astronomy. Show all posts
Showing posts with label Astronomy. Show all posts
Earth
Earth
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| Earth, our home planet, has oceans of liquid water, and continents that rise above sea level. NASA scientists combined satellite photographs with surface data to create this detailed image of Earth's land masses and oceans. The swirling mass of clouds west of Mexico is a large hurricane. Image credit: NASA/Goddard Space Flight Center |
Earth is a small planet in the vastness of space. It is one of nine planets that travel through space around the sun. The sun is a star -- one of billions of stars that make up a galaxy called the Milky Way. The Milky Way and as many as 100 billion other galaxies make up the universe.
The planet Earth is only a tiny part of the universe, but it is the home of human beings and, in fact, all known life in the universe. Animals, plants, and other organisms live almost everywhere on Earth's surface. They can live on Earth because it is just the right distance from the sun. Most living things need the sun's warmth and light for life. If Earth were too close to the sun, it would be too hot for living things. If Earth were too far from the sun, it would be too cold for anything to live. Living things also must have water to live. Earth has plenty. Water covers most of Earth's surface.
The study of Earth is called geology, and scientists who study Earth are geologists. Geologists study different physical features of Earth to understand how they were formed and how they may have changed over time. Much of Earth, such as the deep interior, cannot be studied directly. Geologists must often study samples of rock and use indirect methods to learn about the planet. Today, geologists can also view and study the entire Earth from space.
This article discusses Earth (Earth as a planet) (Earth's spheres) (Earth's rocks) (Cycles on and in Earth) (Earth's interior) (Earth's crust) (Earth's changing climate) (History of Earth).
Earth as a planet
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| The sun is much larger than Earth. From the sun's center to its surface, it is about 109 times the radius of Earth. Some of the streams of gas rising from the solar surface are larger than Earth. Image credit: World Book illustration by Roberta Polfus |
Earth, like all the planets in our solar system, travels around the sun in a path called an orbit. Earth is about 93 million miles (150 million kilometers) from the sun. It takes one year for Earth to complete one orbit around the sun. The innermost planet, Mercury, is only about one-third as far from the sun as Earth and circles the sun in only 88 days. Pluto, the outermost planet, is 40 times as far from the sun as Earth and takes 248 Earth years to circle the sun.
How Earth moves
Earth has three motions. It (1) spins like a top around an imaginary line called an axis that runs from the North Pole to the South Pole, (2) it travels around the sun, and (3) it moves through the Milky Way along with the sun and the rest of the solar system.
Earth takes 24 hours to spin completely around on its axis so that the sun is in the same place in the sky. This period is called a solar day. During a solar day, Earth moves a little around its orbit so that it faces the stars a little differently each night. Thus, it only takes 23 hours 56 minutes 4.09 seconds for Earth to spin once so that the stars appear to be in the same place in the sky. This period is called a sidereal day. A sidereal day is shorter than a solar day, so the stars appear to rise about 4 minutes earlier each day.
Earth takes 365 days 6 hours 9 minutes 9.54 seconds to circle the sun. This length of time is called a sidereal year. Because Earth does not spin a whole number of times as it goes around the sun, the calendar gets out of step with the seasons by about 6 hours each year. Every four years, a day is added to bring the calendar back into line with the seasons. These years, called leap years, have 366 days. The extra day is added to the end of February and occurs as February 29.
The distance around Earth's orbit is 584 million miles (940 million kilometers). Earth travels in its orbit at 66,700 miles (107,000 kilometers) an hour, or 18.5 miles (30 kilometers) a second. Earth's orbit lies on an imaginary flat surface around the sun called the orbital plane.
Earth's axis is not straight up and down, but is tilted by about 23 1/2 degrees compared to the orbital plane. This tilt and Earth's motion around the sun causes the change of the seasons. In January, the northern half of Earth tilts away from the sun. Sunlight is spread thinly over the northern half of Earth, and the north experiences winter. At the same time, the sunlight falls intensely on the southern half of Earth, which has summer. By July, Earth has moved to the opposite side of the sun. Now the northern half of Earth tilts toward the sun. Sunlight falls intensely over the northern half of Earth, and the north experiences summer. At the same time, the sunlight falls less intensely on the southern half of Earth, which has winter.
Earth's orbit is not a perfect circle. Earth is slightly closer to the sun in early January (winter in the Northern Hemisphere) and farther away in July. In January, Earth is 91.4 million miles (147.1 million kilometers) from the sun, and in July it is 94.5 million miles (152.1 million kilometers) from the sun. This variation has a far smaller effect than the heating and cooling caused by the tilt of Earth's axis.
Earth and the solar system are part of a vast disk of stars called the Milky Way Galaxy. Just as the moon orbits Earth and planets orbit the sun, the sun and other stars orbit the tightly packed center of the Milky Way. The solar system is about two-fifths of the way from the center of the Milky Way and revolves around the center at about 155 miles (249 kilometers) per second. The solar system makes one complete revolution around the center of the galaxy in about 220 million years.
Earth's size and shape
Most people picture Earth as a ball with the North Pole at the top and the South Pole at the bottom. Earth, other planets, large moons, and stars -- in fact, most objects in space bigger than about 200 miles (320 kilometers) in diameter -- are round because of their gravity. Gravity pulls matter in toward the center of objects. Tiny moons, such as the two moons of Mars, have so little gravity that they do not become round, but remain lumpy instead.
To our bodies, "down" is always the direction gravity is pulling. People everywhere on Earth feel "down" is toward the center of Earth and "up" is toward the sky. People in Spain and in New Zealand are on exactly opposite sides of Earth from each other, but both sense their surroundings as "right side up." Gravity works the same way on other planets and moons.
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| Earth has a diameter of about 7,900 miles (12,700 kilometers). The diameter of Jupiter, the biggest planet in our solar system, is more than 11 times as large as the diameter of Earth. Image credit: NASA/NSSDC |
Earth's bulge also makes the circumference of Earth larger around the equator than around the poles. The circumference around the equator is 24,901.55 miles (40,075.16 kilometers), but around the poles it is only 24,859.82 miles (40,008.00 kilometers). The circumference is actually greatest just south of the equator, so Earth is slightly pear-shaped. Earth also has mountains and valleys, but these features are tiny compared to the total size of Earth, so the planet appears smooth from space.
Earth and its moon
Earth has one moon. Pluto also has one moon, while Mercury and Venus have none. All the other planets in our solar system have two or more moons. Earth's moon has a diameter of 2,159 miles (3,474 kilometers) -- about one-fourth of Earth's diameter.
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| View of Earth and the moon from space. Image credit: NASA |
Earth's spheres
Earth is composed of several layers, or spheres, somewhat like the layers of an onion. The solid Earth consists of a thin outer layer, the crust, with a thick rocky layer, the mantle, beneath it. The crust and the upper portion of the mantle are called the lithosphere. At the center of Earth is the core. The outer part of the core is liquid, while the inner part is solid. Much of Earth is covered by a layer of water or ice called the hydrosphere. Earth is surrounded by a thin layer of air, the atmosphere. The portion of the hydrosphere, atmosphere, and solid land where life exists is called the biosphere.
The atmosphere
Air surrounds Earth and becomes progressively thinner farther from the surface. Most people find it difficult to breathe more than 2 miles (3 kilometers) above sea level. About 100 miles (160 kilometers) above the surface, the air is so thin that satellites can travel without much resistance. Detectable traces of atmosphere, however, can be found as high as 370 miles (600 kilometers) above Earth's surface. The atmosphere has no definite outer edge but fades gradually into space.
Nitrogen makes up 78 percent of the atmosphere, while oxygen makes up 21 percent. The remaining 1 percent consists of argon and small amounts of other gases. The atmosphere also contains water vapor, carbon dioxide, water droplets, dust particles, and small amounts of many other chemicals released by volcanoes, fires, living things, and human activities.
The lowest layer of the atmosphere is called the troposphere. This layer is in constant motion. The sun heats Earth's surface and the air above it, causing warm air to rise. As the warm air rises, air pressure decreases and the air expands and cools. The cool air is denser than the surrounding air, so it sinks and the cycle starts again. This constant cycle of the air causes the weather.
High above the troposphere, about 30 miles (48 kilometers) above Earth's surface, is a layer of still air called the stratosphere. The stratosphere contains a layer where ultraviolet light from the sun strikes oxygen molecules to create a gas called ozone. Ozone blocks most of the harmful ultraviolet rays from reaching Earth's surface. Some ultraviolet rays get through, however. They are responsible for sunburn and can cause skin cancer in people. Tiny amounts of human-made chemicals have caused some of the natural ozone to break down. Many people are concerned that the ozone layer may become too thin, allowing ultraviolet rays to reach the surface and harm people and other living things.
Water vapor, carbon dioxide, methane, and other gases in the atmosphere trap heat from the sun, warming Earth. The heat-trapping quality of these gases causes the greenhouse effect. Without the greenhouse effect of the atmosphere, Earth would probably be too cold for life to exist.
Ocean waters cover most of Earth's surface. This satellite view shows the Indian Ocean, partly bordered by Africa, Asia, and Australia, and below it the Southern Ocean surrounding Antarctica.
The hydrosphere
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| Ocean waters cover most of Earth's surface. This satellite view shows the Indian Ocean, partly bordered by Africa, Asia, and Australia, and below it the Southern Ocean surrounding Antarctica. Image credit: NASA/Goddard Space Flight Center and ORBIMAGE/SeaWiFS Project |
About 71 percent of Earth's surface is covered by water, most of it in the oceans. Ocean water is too salty to drink. Only about 3 percent of Earth's water is fresh water, suitable for drinking. Much of Earth's fresh water is not readily available to people because it is frozen in the polar ice caps or beneath Earth's surface. Polar regions and high mountains stay cold enough for water to remain permanently frozen. The region of permanent ice on Earth is sometimes called the cryosphere.
The lithosphere
The crust and upper mantle of Earth from the surface to about 60 miles (100 kilometers) down make up the lithosphere. The thin crust is made up of natural chemicals called minerals composed of different combinations of elements. Oxygen is the most abundant chemical element in rocks in Earth's crust, making up about 47 percent of the weight of all rock. The second most abundant element is silicon, 27 percent, followed by aluminum (8 percent), iron (5 percent), calcium (4 percent), and sodium, potassium, and magnesium (about 2 percent each). These eight elements make up 99 percent of the weight of rocks on Earth's surface.
Two elements, silicon and oxygen, make up almost three-fourths of the crust. This combination of elements is so important that geologists have a special term for it: silica. Minerals that contain silica are called silicate minerals. The most abundant mineral on Earth's surface is quartz, made up of pure silica. Another plentiful group of silicates are the feldspars, which consist of silica, aluminum, calcium, sodium, and potassium. Other common silicate minerals on Earth's surface are pyroxene (PY rahk seen) and amphibole (AM fuh bohl), which consist of combinations of silica, iron, and magnesium.
Another important group of minerals are the carbonates, which contain carbon and oxygen along with small amounts of other elements. The most important carbonate mineral is calcite, made up of calcium, carbon, and oxygen. Limestone, a common rock used for building, is mostly calcite. Another important carbonate is dolomite, composed of carbon, oxygen, calcium, and magnesium.
Earth has two kinds of crust. The dry land of the continents is made up mostly of granite and other light silicate minerals, while the ocean floors are composed mostly of a dark, dense volcanic rock called basalt. Continental crust averages about 25 miles (40 kilometers) thick, but it is thicker in some areas and thinner in others. Most oceanic crust is only about 5 miles (8 kilometers) thick. Water fills in the low areas over the thin basalt crust to form the world's oceans. There is more than enough water on Earth to completely fill the oceanic basins, and some of it spreads onto the edges of the continents. This portion of the continents surrounded by a band of shallow ocean is called the continental shelf.
The biosphere
Earth is the only planet in the universe known to have life. The region containing life extends from the bottom of the deepest ocean to a few miles or kilometers into the atmosphere. There are several million known kinds, called species, of living things, and scientists believe that there are far many more species not yet discovered.
Life affects Earth in many ways. Life has actually made the atmosphere around us. Plants take in water and carbon dioxide, both of which contain oxygen. They use the carbon in carbon dioxide and the hydrogen in water to make chemicals of many kinds and give off oxygen as a waste product. Animals eat plants to get energy and return water and carbon dioxide back into the environment. Living things affect the surface of Earth in other ways as well. Plants create chemicals that speed the breakdown of rock. Grasslands and forests slow the erosion of soil.
Earth's rocks
The solid part of Earth consists of rocks, which are sometimes made up of a single mineral, but more often consist of mixtures of minerals. Geologists classify rocks according to their origin. Igneous rocks form when molten rock cools and solidifies. Sedimentary rocks form when grains of rock or dissolved chemicals are deposited in layers by wind, water, or glaciers. Over time, the layers harden into solid rock. Metamorphic rocks develop deep in Earth's crust when heat or pressure transform other types of rock.
Igneous rocks form from molten material called magma. Most of Earth's interior is solid, not molten, but it is extremely hot. At the base of Earth's crust, the temperature is about 1800 degrees F (1000 degrees C). In some portions of the crust, conditions are right for rocks to melt. Rocks can melt more easily near the crust if they contain water, which lowers their melting point.
Where conditions are right, small pockets of magma form beneath and within the crust. Some of this magma reaches the surface, where it erupts from volcanoes as lava. Igneous rocks formed this way are called volcanic or extrusive. Vast quantities of magma, however, never reach the surface. They cool slowly within the crust and may only be exposed long afterward by erosion. Such igneous rocks are called plutonic or intrusive. Plutonic rocks cool slowly. During this slow cooling, their minerals form large crystals. Plutonic rocks tend to be much coarser than volcanic rocks.
Igneous rocks that are rich in silica tend to be poor in iron and magnesium, and the opposite is also true. Volcanic rocks that are iron-rich and silica-poor are basalt. Plutonic rocks of the same makeup are called gabbro. Silica-rich volcanic rocks are called rhyolite (RY uh lyt), and plutonic rocks of the same composition are granite. Granite lies under most of the continents, while basalt lies under most of the ocean floors.
Sedimentary rocks
Rocks on Earth's surface are under constant attack by chemicals and mechanical forces. The processes that break down rocks are called weathering. Water is effective at dissolving minerals. When water freezes, it expands, so expanding ice helps pry apart mineral grains in rocks. In addition, living things produce chemicals that help dissolve rocks.
Once rocks break apart, the loose material is often carried away by erosion. Running water erodes rocks. Wind and glaciers also contribute to erosion. Erosion is usually a relatively slow process, but over millions of years, erosion can uncover even rocks many miles or kilometers below the surface.
Materials derived from weathering and erosion of rocks are eventually deposited to form sedimentary rocks. Rocks that are made up of small pieces of other rocks are called clastic rocks. Rocks containing larger pebbles are called conglomerate. The particles in these rocks are cemented together when minerals dissolved in the water crystallize between the grains. The most abundant sedimentary rocks, called mudrocks, consist of tiny particles. Some of these rocks, called shale, split into thin sheets when broken. Sandstone is a sedimentary rock made up of sand cemented together.
Other sedimentary rocks form when dissolved materials undergo chemical reactions and settle out as tiny solid particles. These rocks are called chemical sedimentary rocks. Common chemical sedimentary rocks include some types of limestone and dolomite. Some chemical sedimentary rocks form when water evaporates, leaving dissolved materials behind. Rock salt and a mineral called gypsum form this way.
Some sedimentary rocks, called biogenic, are formed by the action of living things. Coal is the remains of woody plants that have been transformed into rock by heat and pressure over time. Most limestone is formed by microscopic marine organisms that secrete protective shells of calcium carbonate. When the animals die, the shells remain and solidify into limestone.
Metamorphic rocks
When rocks are buried deeply, they become hot. Earth's crust grows hotter by about 70 degrees F per mile (25 degrees C per kilometer) of depth. Pressure also increases with depth. At a depth of 1 mile (1.6 kilometers) beneath the surface, the pressure is about 6,000 pounds per square inch (41,360 kilopascals). As rocks are heated and subjected to pressure, minerals react and the rocks become metamorphic. Shale is transformed to slate, limestone, and eventually into marble under pressure. Many metamorphic rocks contain recognizable features that tell of their origin, but others change so much that only the chemical makeup provides evidence of what they originally were.
Cycles on and in Earth
Earth can be thought of as a huge system of interacting cycles. In each cycle, matter and energy move from place to place and may change form. Eventually, matter and energy return to their original condition and the cycle begins again. The cycles affect everything on the planet, from the weather to the shape of the landscape. There are many cycles on and within Earth. A few of the most important are (1) atmospheric circulation, (2) ocean currents, (3) the global heat conveyor, (4) the hydrologic cycle, and (5) the rock cycle.
Atmospheric circulation
Air warmed by the sun near the equator rises and flows toward Earth's poles, returning to the surface and flowing back to the equator. This motion, combined with the rotation of Earth, moves heat and moisture around the planet creating winds and weather patterns.
In some areas, the winds change directions with the seasons. These patterns are often called monsoons. In summer, air over Asia is heated by the sun, rises, and draws moist air from the Indian Ocean, causing daily rains over most of southern Asia. In winter, the air over Asia cools, sinks, and flows out, pushing the moist air away and creating dry weather. A similar pattern occurs in the Pacific Ocean near Mexico and brings moist air and afternoon thunderstorms to the southwestern United States in the summer.
Ocean currents are driven by the winds and follow the same general pattern. The continents block the flow of water around the globe, so ocean currents flow west near the equator, then turn toward the poles when they strike a continent, turn east, then flow back to the equator on the other side. In all the oceans, the ocean currents form great loops called gyres. The gyres flow clockwise north of the equator and counterclockwise south of it.
The global heat conveyor is an enormous cycle of ocean water that distributes the oceans' heat around Earth. Water in the polar regions is very cold, salty, and dense. It sinks and flows along the sea floor toward the equator. Eventually, the water rises along the margins of the continents and merges with the surface water flow. When it reaches the polar regions, it sinks again. This three-dimensional movement of water mixes heat throughout the oceans, warming polar waters. It also brings nutrients up from the deep ocean to the surface, where they are available for marine plants and animals.
The hydrologic cycle
Water from the oceans evaporates and is carried by the atmosphere, eventually falling as rain or snow. Water that falls on the land helps break rocks down chemically, nourishes plants, and wears down the landscape. Eventually, the water returns to the sea to start the cycle over again.
The rock cycle
Earth has many more kinds of rocks compared to other planets because there are so many processes acting to form and break down rocks. Geologists sometimes speak of the rock cycle to explain how different rock types are related. The cycle may begin with a flow of lava from a volcano cooling to form new igneous rocks on Earth's surface. As the rock is exposed to water, it breaks down and the resulting materials may be carried away to be deposited as sedimentary rocks. These rocks may eventually be so deeply buried that they change in form to become metamorphic rocks. They may even melt, creating the raw material for the next generation of igneous rocks.
Rocks rarely go through the entire rock cycle. Instead, some steps may be skipped or repeated. For example, igneous rocks can be subjected to heat and pressure and transformed directly to metamorphic rocks. Sedimentary rocks can be broken down by weathering and then reassembled into a new generation of sedimentary rocks. Metamorphic rocks can also be weathered to form the raw material for a new generation of sedimentary rocks. Any rock type, igneous, metamorphic, or sedimentary, can be transformed into any other type.
Earth's interior
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| Beneath Earth's solid crust are the mantle, the outer core, and the inner core. Scientists learn about the inside of Earth by studying how waves from earthquakes travel through the planet. Image credit: World Book illustration by Raymond Perlman and Steven Brayfield, Artisan-Chicago |
One way geologists determine the overall composition of Earth is from chemical analysis of meteorites. Certain types of meteorites, called chondrites, are remains of the early solar system that persisted unchanged in space until they fell to Earth. Geologists can use chondrites to estimate the original chemical composition of the entire Earth.
Unlike chondrites, Earth is made up of layers that contain different amounts of various chemical elements. Geologists learn about Earth's interior by studying vibrations generated by earthquakes, using instruments called seismographs. The speed and motion of vibrations traveling through Earth depends on the composition and density of the material they travel through. Geologists can determine many properties of Earth's interior by analyzing such vibrations.
The mantle
Beneath the crust, extending down about 1,800 miles (2,900 kilometers), is a thick layer called the mantle. The mantle is not perfectly stiff but can flow slowly. Earth's crust floats on the mantle much as a board floats in water. Just as a thick board would rise above the water higher than a thin one, the thick continental crust rises higher than the thin oceanic crust. The slow motion of rock in the mantle moves the continents around and causes earthquakes, volcanoes, and the formation of mountain ranges.
The core
At the center of Earth is the core. The core is made mostly of iron and nickel and possibly smaller amounts of lighter elements, including sulfur and oxygen. The core is about 4,400 miles (7,100 kilometers) in diameter, slightly larger than half the diameter of Earth and about the size of Mars. The outermost 1,400 miles (2,250 kilometers) of the core are liquid. Currents flowing in the core are thought to generate Earth's magnetic field. Geologists believe the innermost part of the core, about 1,600 miles (2,600 kilometers) in diameter, is made of a similar material as the outer core, but it is solid. The inner core is about four-fifths as big as Earth's moon.
Earth gets hotter toward the center. At the bottom of the continental crust, the temperature is about 1800 degrees F (1000 degrees C). The temperature increases about 3 degrees F per mile (1 degrees C per kilometer) below the crust. Geologists believe the temperature of Earth's outer core is about 6700 to 7800 degrees F (3700 to 4300 degrees C). The inner core may be as hot as 12,600 degrees F (7000 degrees C) -- hotter than the surface of the sun. But, because it is under great pressures, the rock in the center of Earth remains solid.
Earth's crust
The hot rock deep in Earth's mantle flows upward slowly, while cooler rock near the surface sinks because hot materials are lighter than cool materials. The rising and sinking of materials due to differences in temperature is called convection. As Earth's mantle flows, it breaks the crust into a number of large slabs called tectonic plates, much as slabs of ice break apart on a pond. The slow flow of Earth's mantle drags the crust along, causing the continents to move, mountains to form, and volcanoes and earthquakes to occur. This constant motion of Earth's crust is called plate tectonics.
In some places, usually under the oceans, Earth's plates are spreading apart. New magma from the mantle rises to fill the cracks between the plates. Places where plates spread apart are called spreading centers. Many volcanoes occur where plates pull apart and magma wells up from within the mantle to fill the gap. The material from the mantle is made of iron and magnesium-rich silicate rocks. It hardens to form rocks and creates oceanic crust made of basalt.
Subduction
Earth's crust cannot spread apart everywhere. Somewhere, an equal amount of crust must be removed. When two plates push together, one of the plates sinks back into Earth's mantle, a process called subduction. The sinking plate eventually melts into magma in Earth's interior. Much of the magma created in subduction zones does not reach the surface and cools within the crust, forming plutonic rocks. The heat from the magma also helps create metamorphic rocks.
Because continental crust is too thick and light to sink into Earth's interior, only plates made of dense oceanic crust are subducted. The boundary where the two plates meet is marked by a deep trench on the ocean floor. The trenches are the deepest places in the oceans, up to 36,000 feet (11,000 meters) deep.
The upper plate that remains on the surface may be continental crust or oceanic crust. This plate is also changed by subduction. As the two plates move together, the edge of the upper plate is compressed. The crust becomes thicker and higher, creating a mountain range. When the rocks of the sinking plate reach a depth of about 60 miles (100 kilometers), they begin to melt and form magma. Some of the magma reaches the surface to form volcanoes. Regions with many volcanoes, such as Peru, Japan, and the northwestern United States, lie near areas where subduction is happening.
Mountain building
Occasionally, as a plate sinks into Earth's mantle, it drags along a continent or a smaller land mass. Continental crust is too thick and light to sink. Instead, it collides with the opposing plate. If the opposing plate is also a continent, neither plate will sink. This type of collision often forms a vast mountain chain in the middle of a continent. The Himalaya were formed in such a way from the collision of two plates of continental crust.
The series of events that happen during formation of a mountain range is called orogeny. Orogeny includes the elevation of mountains, folding and crumpling of the rocks, volcanic activity, and formation of plutonic and metamorphic rocks that occur when plates collide. Long after mountains have vanished from erosion, geologists can still see the changes orogeny produces in the rocks.
Terrane collisions
Smaller pieces of continental crust that collide with another plate are often added to the edge of the larger plate. These small added pieces of crust are called terranes. Most of the land in the United States west of Salt Lake City has been added to North America by terrane collisions in the last 500 million years.
Earthquakes
Earthquakes occur when rocks on opposite sides of a break in the crust, called a fault, slide past each other. The boundaries between plates are faults, but there are faults within plates as well. Occasionally, forces within the plates cause rocks to fracture and slip even though the rocks are not at a plate boundary. The boundaries between two plates sliding past each other are called transform faults. The San Andreas Fault in California is a transform fault, where a portion of crust called the Pacific Plate is carrying a small piece of California northwest past the rest of North America.
The shaping of the continents
Several times in Earth's history, collisions between continents have created a huge supercontinent. Although the crust of the continents is thick, it breaks more easily than oceanic crust, and supercontinents broke quickly into smaller pieces. Material from Earth's mantle filled the gaps, creating new oceanic crust. As the continents moved apart, new ocean basins formed between them. About one-third of Earth's surface is covered by continental crust, so the pieces cannot move far before colliding. As two continents collide, an old ocean basin is destroyed. The process of continents breaking apart and rejoining is called the Wilson cycle, after the Canadian geologist John Tuzo Wilson, who first described it.
The continents have probably been in motion for at least the past 2 billion years or more. Geologists, however, only have evidence from rocks to understand and reconstruct the motion over the past 800 million years. Most of the oceanic crust older than that has been subducted into the mantle long ago.
Geologists have determined that, about 800 million years ago, the continents were assembled into a large supercontinent called Rodinia. What is now North America lay at the center of Rodinia. The flow of material in Earth's mantle caused Rodinia to break apart into many pieces, which collided again between 500 million and 250 million years ago. Collision between what is now North America, Europe, and Africa caused the uplift of the Appalachian Mountains in North America. Collisions between part of present-day Siberia and Europe created the Ural Mountains.
By 250 million years ago, the continents reassembled to form another supercontinent called Pangaea. A single, worldwide ocean, called Panthalassa, surrounded Pangaea. About 200 million years ago, Pangaea began to break apart. It split into two large land masses called Gondwanaland and Laurasia. Gondwanaland then broke apart, forming the continents of Africa, Antarctica, Australia, and South America, and the Indian subcontinent. Laurasia eventually split apart into Eurasia and North America. As the continental plates split and drifted apart, new oceanic crust formed between them. The movement of the continents to their present positions took place over millions of years.
Earth's changing climate
The ice ages
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| Precambrian time included almost all of Earth's first 4 billion years. The crust, the atmosphere, and the oceans were formed, and the simplest kinds of life appeared. Image credit: World Book illustration by Ian Jackson, WILDlife Art |
Most of the time, Earth has been largely ice free. Brief ice ages occurred about 450 million years ago and again about 250 million years ago. In the last few million years, however, Earth's climate began to cool. Glaciers began forming in Antarctica about 35 million years ago, but the climate there was warm enough for trees to grow until about 5 million years ago. By about 2 million years ago, at the beginning of a time called the Pleistocene Epoch, ice had accumulated on other continents as well.
Numerous separate ice advances, periods when ice sheets covered vast areas, occurred during the Pleistocene Ice Age. The advances alternated with periods when the climate was warmer and the ice melted. Geologists analyzing sediment deposits from the North Atlantic Ocean determined that there were at least 20 advances and retreats of ice sheets in the past 2 million years. At least four ice advances were big enough to extend over much of Europe, cover most of Canada, and reach deep into the United States.
The most recent advance of ice began about 70,000 years ago and reached its farthest extent about 18,000 years ago. The vast glaciers and sheets of ice scoured out the basins of the Great Lakes and blocked rivers, completely changing the courses of the Mississippi, Missouri, and Ohio rivers. So much water was trapped in the form of ice that sea level around Earth dropped as much as 390 feet (120 meters), exposing parts of the present ocean floor.
The most recent ice advance ended about 11,500 years ago. Most scientists believe that Earth is currently in an interglacial period, and another ice advance will follow.
Why ice ages occur
Scientists do not fully understand why Earth has ice ages. Most believe that tiny changes in Earth's orbit and axis due to the gravitational pull of other planets play a part. These changes alter the amount of energy received from the sun.
Many scientists also believe that variations in the amount of carbon dioxide in the atmosphere are responsible for long-term changes in the climate. Carbon dioxide, a "greenhouse gas," traps heat from the sun and warms Earth's atmosphere. Most of Earth's carbon dioxide is locked in carbonate rocks, such as limestone and dolomite. Earth's climate today would be much warmer if the carbon dioxide trapped in limestone were released into the atmosphere.
When mountains rich in silicate minerals wear down through weathering and erosion, calcium and magnesium erode from the rocks. These elements are carried to the sea by water. There, living organisms absorb the chemicals and use them to make protective carbonate shells. The organisms eventually die and sink to the bottom to form limestone deposits. This process, called the carbonate-silicate cycle, removes carbon dioxide from the atmosphere. With less carbon dioxide in the atmosphere to trap heat from the sun, Earth's climate may cool enough to cause an ice age.
Limestone and dolomite deposits exposed to weathering and erosion return carbon dioxide to the atmosphere and contribute to global warming. In addition, some limestone on the ocean floor can be carried down into Earth's mantle by subduction. Beneath the crust, the limestone breaks down into magma under heat and pressure. The carbon dioxide in the limestone can then return to the atmosphere during volcanic eruptions.
Scientists theorize that volcanoes continued to emit carbon dioxide into the atmosphere during the Precambrian ice ages. Eventually, the carbon dioxide warmed Earth through the greenhouse effect, causing the ice to melt rapidly.
History of Earth
The history of Earth is recorded in the rocks of Earth's crust. Rocks have been forming, wearing away, and re-forming ever since Earth took shape. The products of weathering and erosion are called sediment. Sediment accumulates in layers known as strata. Strata contain clues that tell geologists about Earth's past. These clues include the composition of the sediment, the way the strata are deposited, and the kinds of fossils that may occur in the rock.
Space exploration has expanded our understanding of Earth's origin. The Hubble Space Telescope has observed what appear to be stars in the process of forming planets. Since the mid-1990's, scientists have found other stars that have planets surrounding them. These discoveries have helped scientists develop theories about the formation of Earth.
Age of Earth
Scientists think that Earth probably formed at about the same time as the rest of the solar system. They have determined that some chondrite meteorites, the unaltered remains from the formation of the solar system, are up to 4.6 billion years old. Scientists believe that Earth and other planets are probably that old. They can determine the ages of rocks by measuring the amounts of natural radioactive materials, such as uranium, in them. Radioactive elements decay (change into other elements) at a known rate. For example, uranium gives off radiation and decays into lead. Scientists know the time it takes for uranium to change to lead. They can determine the age of a rock by comparing the amount of uranium to the amount of lead.
The known history of Earth is divided into four long stretches of time called eons. Starting with the earliest, the eons are Hadean, Archean, Proterozoic, and Phanerozoic. The first three eons, which together lasted nearly 4 billion years, are grouped into a unit called the Precambrian. The Phanerozoic Eon, when life became abundant, is divided into three eras. They are, from the oldest to the youngest, the Paleozoic, Mesozoic, and Cenozoic eras. Eras are divided into periods, and periods are divided into epochs. These divisions and subdivisions are named for places where rocks of each period were studied. Periods are mostly separated by important changes in the types of fossils found in the rocks. As a result, the lengths of eras, periods, and epochs are not equal.
A chart showing an outline of Earth's history is called a geological time scale. On such a chart, Earth's earliest history is at the bottom, and its recent history at the top. This arrangement resembles the way rock strata are formed, with the recent over the oldest.
Formation of Earth
Most scientists believe that the solar system began as a thin cloud of gas and dust in space. The sun itself may have formed from a portion of the cloud that was thicker than the rest. The cloud's own gravity caused it to start contracting, and dust and gas were drawn in toward the center. Much of the cloud collapsed to the center to form a star, the sun, but a great ring of material remained orbiting around the star. Particles in the ring collided to make larger objects, which in turn collided to build up the planets of the solar system in a process called accretion. Scientists believe that many small planets formed and then collided to make larger planets.
Earth's early development
Scientists theorize that Earth began as a waterless mass of rock surrounded by a cloud of gas. Radioactive materials in the rock and increasing pressure in Earth's interior produced enough heat to melt the interior of Earth. The heavy materials, such as iron, sank. The light silicate rocks rose to Earth's surface and formed the earliest crust. The heat of the interior caused other chemicals inside Earth to rise to the surface. Some of these chemicals formed water, and others became the gases of the atmosphere.
In 2001, an international team of scientists announced the discovery of crystals of the mineral zircon that they determined to be 4.4 billion years old. Zircon, made up of the elements zirconium, silicon, and oxygen, is a hard, long lasting mineral that resists erosion and weathering. Through chemical analysis of the zircon, the scientists determined that liquid water probably existed on Earth's surface when the crystal were formed. They concluded that Earth's crust and oceans may have formed within about 200 million years after the planet had taken shape.
Astronomers believe that the sun was about 30 percent fainter when Earth first formed than it is today. The oldest rocks on Earth, however, provide evidence that Earth was warm enough for liquid water to exist on the surface. Scientists believe that the atmosphere must have been thicker than it is today, to trap more heat from the sun. Over millions of years, the water slowly collected in low places of the crust and formed oceans.
After the main period of planet formation, most of the remaining debris in the solar system was swept up by the newly formed planets. The collisions of the newly formed planets and debris material were explosive. The impacts created the cratered surfaces of the moon, Mars, Venus, and Mercury. Earth was also struck, but the craters produced by the impacts have all been destroyed by erosion and plate tectonics. Geologists believe that large masses of continental crust had formed by 3.5 billion years ago. There is evidence that plate tectonics has been active for at least 2 billion years.
Some scientists believe Earth's early atmosphere contained hydrogen, helium, methane, and ammonia, much like the present atmosphere of Jupiter. Others believe it may have contained a large amount of carbon dioxide, as does the atmosphere of Venus. Scientists agree that Earth's earliest atmosphere probably had little oxygen.
Geologists have determined that, about 2 billion years ago, a change in Earth's atmosphere occurred. They know this because certain kinds of iron ores created in oxygen-poor environments stopped forming at that time. Instead, large deposits of red sandstone formed. The red color results from iron reacting with oxygen to form iron oxide, or rust. The sandstone deposits are evidence that Earth's atmosphere contained some oxygen. The air was not breathable at that time, but the atmosphere may have had about 1 percent oxygen.
The oxygen in the atmosphere today comes mainly from plants and microorganisms such as algae. These organisms use carbon dioxide and give off oxygen through the process of photosynthesis. The amount of oxygen increased in the atmosphere of the early Earth as oxygen-producing organisms developed and became more plentiful.
Life on Earth
Many rocks contain fossils that reveal the history of life on Earth. A fossil may be an animal's body, a tooth, or a piece of bone. It may simply be an impression of a plant or an animal made in a rock when the rock was soft sediment. Fossils help scientists learn which kinds of plants and animals lived at different times in Earth's history. Scientists who study prehistoric life are called paleontologists.
Many scientists believe that life appeared on Earth almost as soon as conditions allowed. There is evidence for chemicals created by living things in rocks from the Archean age, 3.8 billion years old. Fossil remains of microscopic living things about 3.5 billion years old have also been found at sites in Australia and Canada.
For most of Earth's history, life consisted mainly of microscopic, single-celled creatures. The earliest fossils of larger creatures with many cells are found in Precambrian rocks that are about 600 million years old. Many of these creatures differed from any living things today.
The Paleozoic Era
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| The Paleozoic Era saw the development of many kinds of animals and plants in the seas and on land. The earliest land plants appeared in the Silurian Period, about 440 million years ago. Image credit: World Book illustration by Ian Jackson, WILDlife Art |
Most fossil organisms found in Paleozoic rocks are invertebrates (animals without a backbone), such as corals, mollusks (clams and snails), and trilobites (flat-shelled sea animals). Fish, the earliest vertebrates (animals with a backbone), are first found in Ordovician rocks about 450 million years old. Silurian rocks, about 440 million years old, contain fossils of the first large land plants. Amphibians, animals capable of living on land or in the water, first appear as fossils in Devonian rocks about 380 million years old.
Fossil remains preserved in rocks show that by 300 million years ago, large forests and swamps covered the land. The carbon-rich remains of some of these forests are preserved as coal deposits in the United States, Canada, the United Kingdom, and other parts of the world. The Carboniferous Period is named for these enormous deposits of coal.
The earliest fossil remains of reptiles are found in rocks of the Carboniferous Period. Unlike amphibians, reptiles have scaly skins that keep them from drying out, and they lay eggs protected by a shell. These features enable reptiles to live their whole lives out of water. Toward the end of the Paleozoic Era, in rocks from the Permian Period, some fossil reptiles begin to show some characteristics of mammals.
Several times in Earth's history, there have been great extinctions, periods when many of Earth's living things die out. The greatest of these events, called the Permian extinction, happened about 250 million years ago. Almost 90 percent of the species on Earth during the Permian became extinct in a relatively short time. The cause of this event is a mystery, though many scientists suspect that huge volcanic eruptions in what is now Siberia may have disturbed the climate, causing many organisms to die out.
The Mesozoic Era
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| The Mesozoic Era was the Age of Dinosaurs. Plant-eating dinosaurs, such as this Stegosaurus, fed on cycads and conifers, early trees that thrived before modern flowering trees appeared. Image credit: World Book illustration by Ian Jackson, WILDlife Art |
Fossil plants of the Mesozoic Era represent two main groups, gymnosperms and angiosperms. Gymnosperms have naked seeds, and most are cone-bearing. They include conifers, ginkgoes, and cycads. These gymnosperms evolved in the later part of the Paleozoic Era and were dominant into the early Cretaceous Period. Angiosperms have covered seeds and are flowering plants. They became the dominant plant group during the Cretaceous Period and continue to be so today.
The dinosaurs died out in another great extinction about 65 million years ago. Most scientists believe that the extinction was caused by the impact of a small asteroid with Earth. The impact would have thrown so much dust into the atmosphere that the surface would have been dark and cold for months, killing off plants and the animals that fed on them. Many scientists believe a large, buried crater in the Yucatan region of Mexico, called Chicxulub (CHEEK shoo loob), is the place the asteroid struck. Debris from the collision has been found all over the world, and deposits created by large sea waves caused by the impact have been found in several places around the Gulf of Mexico.
The Cenozoic Era
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| The Cenozoic Era included the Pleistocene Ice Age, when glaciers swept slowly across large areas before melting. The moving ice created a variety of landscapes in northern lands. Image credit: World Book illustration by Ian Jackson, WILDlife Art |
During the Eocene Epoch, ancestors of the horse, rhinoceros, and camel roamed Europe and North America. By the Oligocene Epoch, dogs and cats had appeared, along with three-toed horses about as large as sheep. The mammals grew larger and developed in greater variety as prairies spread over the land during the Miocene Epoch. By the Pliocene Epoch, many kinds of mammals had grown to gigantic size. Elephantlike mammoths and mastodons and giant ground sloths roamed the prairies and forests. These animals died out at the end of the Pleistocene Epoch.
Fossils of the first humanlike creatures appeared near the beginning of the Pleistocene Epoch, about 2 million years ago. The first true human beings appeared later, perhaps less than 200,000 years ago. Humanity's years on Earth are only a brief moment among the billions of years during which Earth has developed.Contributor: Steven I. Dutch, Ph.D., Professor of Earth Science, Department of Natural and Applied Sciences, University of Wisconsin, Green Bay.
Jupiter
Jupiter
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| The layers of dense clouds around Jupiter appear in a photograph of the planet taken by the Voyager 1 space probe. The large, oval-shaped mark on the clouds is the Great Red Spot. The spot is believed to be an intense atmospheric disturbance. Image credit: Jet Propulsion Laboratory |
Jupiter is the fifth planet from the sun. Its mean (average) distance from the sun is about 483,780,000 miles (778,570,000 kilometers), more than five times Earth's distance. Ancient astronomers named Jupiter after the king of the Roman gods.
Astronomers have studied Jupiter with telescopes based on Earth and aboard artificial satellites in orbit around Earth. In addition, the United States has sent six space probes (crewless exploratory craft) to Jupiter.
Astronomers witnessed a spectacular event in July 1994, when 21 fragments of a comet named Shoemaker-Levy 9 crashed into Jupiter's atmosphere. The impacts caused tremendous explosions, some scattering debris over areas larger than the diameter of Earth.
Physical features of Jupiter
Jupiter is a giant ball of gas and liquid with little, if any, solid surface. Instead, the planet's surface is composed of dense red, brown, yellow, and white clouds. The clouds are arranged in light-colored areas called zones and darker regions called belts that circle the planet parallel to the equator.
Orbit and rotation
Jupiter travels around the sun in a slightly elliptical (oval-shaped) orbit. The planet completes one orbit in 4,333 Earth days, or almost 12 Earth years.
As Jupiter orbits the sun, the planet rotates on its axis, an imaginary line through its center. The axis is tilted about 3¡. Scientists measure tilt relative to a line at a right angle to the orbital plane, an imaginary surface touching all points of the orbit.
Jupiter rotates faster than any other planet. It takes 9 hours 56 minutes to spin around once on its axis, compared with 24 hours for Earth. Scientists cannot measure the rotation of the interior of the giant planet directly, so they have calculated the speed from indirect measurements. They first calculated the speed using an average of the speeds of the visible clouds that move with interior currents, except for a more rapid zone near the equator.
Jupiter sends out radio waves strong enough to be picked up by radio telescopes on Earth. Scientists now measure these waves to calculate Jupiter's rotational speed. The strength of the waves varies under the influence of Jupiter's magnetic field in a pattern that repeats every 9 hours 56 minutes. Because the magnetic field originates in Jupiter's core, this variation shows how fast the plant's interior spins.
Jupiter's rapid rotation makes it bulge at the equator and flatten at the poles. The planet's diameter is about 7 percent larger at the equator than at the poles.
Mass and density
Jupiter is heavier than any other planet. Its mass (quantity of matter) is 318 times larger than that of Earth. Although Jupiter has a large mass, it has a relatively low density. Its density averages 1.33 grams per cubic centimeter, slightly more than the density of water. The density of Jupiter is about 1/4 that of Earth. Because of Jupiter's low density, astronomers believe that the planet consists primarily of hydrogen and helium, the lightest elements. Earth, on the other hand, is made up chiefly of metals and rock. Jupiter's mix of chemical elements resembles that of the sun, rather than that of Earth.
Jupiter may have a core made up of heavy elements. The core may be of about the same chemical composition as Earth, but 20 or 30 times more massive.
The force of gravity at the surface of Jupiter is up to 2.4 times stronger than on Earth. Thus, an object that weighs 100 pounds on Earth would weigh as much as 240 pounds on Jupiter.
The atmosphere of Jupiter is composed of about 86 percent hydrogen, 14 percent helium, and tiny amounts of methane, ammonia, phosphine, water, acetylene, ethane, germanium, and carbon monoxide. The percentage of hydrogen is based on the number of hydrogen molecules in the atmosphere, rather than on their total mass. Scientists have calculated these amounts from measurements taken with telescopes and other instruments on Earth and aboard spacecraft.
These chemicals have formed colorful layers of clouds at different heights. The highest white clouds in the zones are made of crystals of frozen ammonia. Darker, lower clouds of other chemicals occur in the belts. At the lowest levels that can be seen, there are blue clouds. Astronomers had expected to detect water clouds about 44 miles (70 kilometers) below the ammonia clouds. However, none have been discovered at any level.
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| The planet Jupiter's Great Red Spot is a huge mass of swirling gas. At its widest, it is about three times the diameter of the Earth. Image credit: NASA |
The edge of the Great Red Spot circulates at a speed of about 225 miles (360 kilometers) per hour. The spot remains at the same distance from the equator but drifts slowly east and west.
The zones, belts, and the Great Red Spot are much more stable than similar circulation systems on Earth. Since astronomers began to use telescopes to observe these features in the late 1600's, the features have changed size and brightness but have kept the same patterns.
Temperature
The temperature at the top of Jupiter's clouds is about -230 degrees F (-145 degrees C). Measurements made by ground instruments and spacecraft show that Jupiter's temperature increases with depth below the clouds. The temperature reaches 70 degrees F (21 degrees C) -- "room temperature" -- at a level where the atmospheric pressure is about 10 times as great as it is on Earth. Scientists speculate that if Jupiter has any form of life, the life form would reside at this level. Such life would need to be airborne, because there is no solid surface at this location on Jupiter. Scientists have discovered no evidence for life on Jupiter.
Near the planet's center, the temperature is much higher. The core temperature may be about 43,000 degrees F (24,000 degrees C) -- hotter than the surface of the sun.
Jupiter is still losing the heat produced when it became a planet. Most astronomers believe that the sun, the planets, and all the other bodies in the solar system formed from a spinning cloud of gas and dust. The gravitation of the gas and dust particles packed them together into dense clouds and solid chunks of material. By about 4.6 billion years ago, the material had squeezed together to form the various bodies in the solar system. The compression of material produced heat. So much heat was produced when Jupiter formed that the planet still radiates about twice as much heat into space as it receives from sunlight.
Magnetic field
Like Earth and many other planets, Jupiter acts like a giant magnet. The force of its magnetism extends far into space in a region surrounding the planet called its magnetic field. Jupiter's magnetic field is about 14 times as strong as Earth's, according to measurements made by spacecraft. Jupiter's magnetic field is the strongest in the solar system, except for fields associated with sunspots and other small regions on the sun's surface.
Scientists do not fully understand how planets produce magnetic fields. They suspect, however, that the movement of electrically charged particles in the interior of planets generates the fields. Jupiter's field would be so much stronger than Earth's because of Jupiter's greater size and faster rotation.
Jupiter's magnetic field traps electrons, protons, and other electrically charged particles in radiation belts around the planet. The particles are so powerful that they can damage instruments aboard spacecraft operating near the planet.
Within a region of space called the magnetosphere, Jupiter's magnetic field acts as a shield. The field protects the planet from the solar wind, a continuous flow of charged particles from the sun. Most of these particles are electrons and protons traveling at a speed of about 310 miles (500 kilometers) per second. The field traps the charged particles in the radiation belts. The trapped particles enter the magnetosphere near the poles of the magnetic field. On the side of the planet away from the sun, the magnetosphere stretches out into an enormous magnetic tail, often called a magnetotail, that is at least 435 million miles (700 million kilometers) long.
Radio waves given off by Jupiter reach radio telescopes on Earth in two forms -- bursts of radio energy and continuous radiation. Strong bursts occur when Io, the closest of Jupiter's four large moons, passes through certain regions in the planet's magnetic field. Continuous radiation comes from Jupiter's surface as well as from high-energy particles in the radiation belts.
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| Callisto, a moon of Jupiter, is covered with craters produced when asteroids and comets struck its icy surface. Beneath the surface may be an ocean of salty liquid water. Image credit: NASA |
Jupiter has 16 satellites that measure at least 6 miles (10 kilometers) in diameter. It also has many smaller satellites. Jupiter's four largest satellites, in order of their distance from Jupiter, are Io, Europa, Ganymede, and Callisto. These four moons are called the Galilean satellites. The Italian astronomer Galileo discovered them in 1610 with one of the earliest telescopes.
Io has many active volcanoes, which produce gases containing sulfur. The yellow-orange surface of Io probably consists largely of solid sulfur that was deposited by the eruptions. Europa ranks as the smallest of the Galilean satellites, with a diameter of 1,945 miles (3,130 kilometers). Europa has a smooth, cracked, icy surface.
The largest Galilean satellite is Ganymede, with a diameter of 3,273 miles (5,268 kilometers). Ganymede is larger than the planet Mercury. Callisto, with a diameter of 2,986 miles (4,806 kilometers), is slightly smaller than Mercury. Ganymede and Callisto appear to consist of ice and some rocky material. The two satellites have many craters.
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| Ganymede, a moon of Jupiter, has craters and cracks on its surface. Asteroids and comets that hit Ganymede made the craters. The cracks are due to expansion and contraction of the surface. Image credit: NASA |
Rings
Jupiter has three thin rings around its equator. They are much fainter than the rings of Saturn. Jupiter's rings appear to consist mostly of fine dust particles. The main ring is about 20 miles (30 kilometers) thick and more than 4,000 miles (6,400 kilometers) wide. It circles the planet inside the orbit of Amalthea.
The impact of Comet Shoemaker-Levy 9
In March 1993, astronomers Eugene Shoemaker, Carolyn Shoemaker, and David H. Levy discovered a comet near Jupiter. The comet, later named Shoemaker-Levy 9, probably once orbited the sun independently, but had been pulled by Jupiter's gravity into an orbit around the planet. When the comet was discovered, it had broken into 21 pieces. The comet probably had broken apart when it passed close to Jupiter.
Calculations based on the comet's location and velocity showed that the fragments would crash into Jupiter's atmosphere in July 1994. Scientists hoped to learn much about the effects of a collision between a planet and a comet.
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| Scars from the crash of Comet Shoemaker-Levy 9 appear on Jupiter's surface as a series of maroon blotches in this photo. The comet broke into 21 pieces before it hit Jupiter in 1994. Image credit: Hubble Space Telescope Comet Team and NASA |
The fragments fell on the back side of Jupiter as viewed from Earth and the Hubble Space Telescope. But the rotation of Jupiter carried the impact sites around to the visible side after less than half an hour. Scientists estimate that the largest fragments were about 0.3 to 2.5 miles (0.5 to 4 kilometers) in diameter. The impacts were directly observable from Galileo, which was within about 150 million miles (240 million kilometers) from Jupiter. However, damage to certain of the probe's instruments limited its ability to record and send data.
The impacts caused large explosions, probably due to the compression, heating, and rapid expansion of atmospheric gases. The explosions scattered comet debris over large areas, some with diameters larger than that of Earth. The debris gradually spread into a dark haze of fine material that remained suspended for several months in Jupiter's upper atmosphere. If a similar comet ever collided with Earth, it might produce a haze that would cool the atmosphere and darken the planet by absorbing sunlight. If the haze lasted long enough, much of Earth's plant life could die, along with the people and animals that depend on plants.
Flights to Jupiter
The United States has sent six space probes to Jupiter: (1) Pioneer 10, (2) Pioneer-Saturn, (3) Voyager 1, (4) Voyager 2, (5) Ulysses, and (6) Galileo.
Pioneer 10 was launched in 1972 and flew within 81,000 miles (130,000 kilometers) of Jupiter on Dec. 3, 1973. The probe revealed the severe effects of Jupiter's radiation belt on spacecraft. Pioneer 10 also reported the amount of hydrogen and helium in the planet's atmosphere. In addition, the probe discovered that Jupiter has an enormous magnetosphere.
Pioneer-Saturn flew within 27,000 miles (43,000 kilometers) of Jupiter in December 1974. The craft provided close-up photographs of Jupiter's polar regions and data on the Great Red Spot, the magnetic field, and atmospheric temperatures.
Voyager 1 and Voyager 2 flew past Jupiter in March and July 1979, respectively. These craft carried more sensitive instruments than did the Pioneers, and transmitted much more information. Astronomers used photographs taken by the Voyagers to make the first detailed maps of the Galilean satellites. The Voyagers also revealed sulfur volcanoes on Io, discovered lightning in Jupiter's clouds, and mapped flow patterns in the cloud bands.
Ulysses was launched in October 1990 and passed by Jupiter in February 1992. The European Space Agency, an organization of Western European nations, had built the probe mainly to study the sun's polar regions. Scientists used the tremendous gravitational force of Jupiter to put Ulysses into an orbit that would take it over the sun's polar regions. As Ulysses passed by Jupiter, it gathered data indicating that the solar wind has a much greater effect on Jupiter's magnetosphere than earlier measurements had suggested.
Galileo began its journey to Jupiter in October 1989. The craft released an atmospheric probe in July 1995. In December 1995, the probe plunged into Jupiter's atmosphere. The probe penetrated deep into the cloud layers and measured the amount of water and other chemicals in the atmosphere. Also in December 1995, Galileo went into orbit around Jupiter. Over the next several years, the craft monitored Jupiter's atmosphere and observed the planet's major satellites. Galileo's mission was extended in 1997 and again in 1999. Eventually, however, the craft ran low on fuel. In September 2003, mission managers intentionally crashed Galileo into Jupiter's atmosphere to avoid any risk of the craft crashing into and contaminating Jupiter's moon Europa. Galileo's observations of Europa had shown that it might have an ocean below its surface capable of supporting life.Contributors: Peter J. Gierasch, Ph.D., Professor of Astronomy, Cornell University. Philip D. Nicholson, Ph.D., Professor of Astronomy, Cornell University.
Star
Star
A globular cluster is a tightly grouped swarm of stars held together by gravity. This globular cluster is one of the densest of the 147 known clusters in the Milky Way galaxy.
A globular cluster is a tightly grouped swarm of stars held together by gravity. This globular cluster is one of the densest of the 147 known clusters in the Milky Way galaxy. Image credit: NASA
A star is a huge, shining ball in space that produces a tremendous amount of light and other forms of energy. The sun is a star, and it supplies Earth with light and heat energy. The stars look like twinkling points of light -- except for the sun. The sun looks like a ball because it is much closer to Earth than any other star.
The sun and most other stars are made of gas and a hot, gaslike substance known as plasma. But some stars, called white dwarfs and neutron stars, consist of tightly packed atoms or subatomic particles. These stars are therefore much more dense than anything on Earth.
Stars come in many sizes. The sun's radius (distance from its center to its surface) is about 432,000 miles (695,500 kilometers). But astronomers classify the sun as a dwarf because other kinds of stars are much bigger. Some of the stars known as supergiants have a radius about 1,000 times that of the sun. The smallest stars are the neutron stars, some of which have a radius of only about 6 miles (10 kilometers).
About 75 percent of all stars are members of a binary system, a pair of closely spaced stars that orbit each other. The sun is not a member of a binary system. However, its nearest known stellar neighbor, Proxima Centauri, is part of a multiple-star system that also includes Alpha Centauri A and Alpha Centauri B.
The distance from the sun to Proxima Centauri is more than 25 trillion miles (40 trillion kilometers). This distance is so great that light takes 4.2 years to travel between the two stars. Scientists say that Proxima Centauri is 4.2 light-years from the sun. One light-year, the distance that light travels in a vacuum in a year, equals about 5.88 trillion miles (9.46 trillion kilometers).
Stars are grouped in huge structures called galaxies. Telescopes have revealed galaxies throughout the universe at distances of 12 billion to 16 billion light-years. The sun is in a galaxy called the Milky Way that contains more than 100 billion stars. There are more than 100 billion galaxies in the universe, and the average number of stars per galaxy may be 100 billion. Thus, more than 10 billion trillion stars may exist. But if you look at the night sky far from city lights, you can see only about 3,000 of them without using binoculars or a telescope.
Stars, like people, have life cycles -- they are born, pass through several phases, and eventually die. The sun was born about 4.6 billion years ago and will remain much as it is for another 5 billion years. Then it will grow to become a red giant. Late in the sun's lifetime, it will cast off its outer layers. The remaining core, called a white dwarf, will slowly fade to become a black dwarf.
Other stars will end their lives in different ways. Some will not go through a red giant stage. Instead, they will merely cool to become white dwarfs, then black dwarfs. A small percentage of stars will die in spectacular explosions called supernovae.
This article discusses Star (The stars at night) (Names of stars) (Characteristics of stars) (Fusion in stars) (Evolution of stars).
The stars at night
If you look at the stars on a clear night, you will notice that they seem to twinkle and that they differ greatly in brightness. A much slower movement also takes place in the night sky: If you map the location of several stars for a few hours, you will observe that all the stars revolve slowly about a single point in the sky.
Twinkling of stars is caused by movements in Earth's atmosphere. Starlight enters the atmosphere as straight rays. Twinkling occurs because air movements constantly change the path of the light as it comes through the air. You can see a similar effect if you stand in a swimming pool and look down. Unless the water is almost perfectly still, your feet will appear to move and change their shape. This "twinkling" occurs because the moving water constantly changes the path of the light rays that travel from your feet to your eyes.
Brightness of stars. How bright a star looks when viewed from Earth depends on two factors: (1) the actual brightness of the star -- that is, the amount of light energy the star emits (sends out) -- and (2) the distance from Earth to the star. A nearby star that is actually dim can appear brighter than a distant star that is really extremely brilliant. For example, Alpha Centauri A seems to be slightly brighter than a star known as Rigel. But Alpha Centauri A emits only 1/100,000 as much light energy as Rigel. Alpha Centauri A seems brighter because it is only 1/325 as far from Earth as Rigel is -- 4.4 light-years for Alpha Centauri A, 1,400 light-years for Rigel.
Rising and setting of stars
When viewed from Earth's Northern Hemisphere, stars rotate counterclockwise around a point called the celestial north pole. Viewed from the Southern Hemisphere, stars rotate clockwise about the celestial south pole. During the day, the sun moves across the sky in the same direction, and at the same rate, as the stars. These movements do not result from any actual revolution of the sun and stars. Rather, they occur because of the west-to-east rotation of Earth about its own axis. To an observer standing on the ground, Earth seems motionless, while the sun and stars seem to move in circles. But actually, Earth moves.
Names of stars
Ancient people saw that certain stars are arranged in patterns shaped somewhat like human beings, animals, or common objects. Some of these patterns, called constellations, came to represent figures of mythological characters. For example, the constellation Orion (the Hunter) is named after a hero in Greek mythology.
Today, astronomers use constellations, some of which were described by the ancients, in the scientific names of stars. The International Astronomical Union (IAU), the world authority for assigning names to celestial objects, officially recognizes 88 constellations. These constellations cover the entire sky. In most cases, the brightest star in a given constellation has alpha -- the first letter of the Greek alphabet -- as part of its scientific name. For instance, the scientific name for Vega, the brightest star in the constellation Lyra (the Harp), is Alpha Lyrae. Lyrae is Latin for of Lyra.
The second brightest star in a constellation is usually designated beta, the second letter of the Greek alphabet, the third brightest is gamma, and so on. The assignment of Greek letters to stars continues until all the Greek letters are used. Numerical designations follow.
But the number of known stars has become so large that the IAU uses a different system for newly discovered stars. Most new names consist of an abbreviation followed by a group of symbols. The abbreviation stands for either the type of star or a catalog that lists information about the star. For example, PSR J1302-6350 is a type of star known as a pulsar -- hence the PSR in its name. The symbols indicate the star's location in the sky. The 1302 and the 6350 are coordinates that are similar to the longitude and latitude designations used to indicate locations on Earth's surface. The J indicates that a coordinate system known as J2000 is being used.
Characteristics of stars
A star has five main characteristics: (1) brightness, which astronomers describe in terms of magnitude or luminosity; (2) color; (3) surface temperature; (4) size; and (5) mass (amount of matter). These characteristics are related to one another in a complex way. Color depends on surface temperature, and brightness depends on surface temperature and size. Mass affects the rate at which a star of a given size produces energy and so affects surface temperature. To make these relationships easier to understand, astronomers developed a graph called the Hertzsprung-Russell (H-R) diagram. This graph, a version of which appears in this article, also helps astronomers understand and describe the life cycles of stars.
Magnitude and luminosity
Magnitude is based on a numbering system invented by the Greek astronomer Hipparchus in about 125 B.C. Hipparchus numbered groups of stars according to their brightness as viewed from Earth. He called the brightest stars first magnitude stars, the next brightest second magnitude stars, and so on to sixth magnitude stars, the faintest visible stars.
Modern astronomers refer to a star's brightness as viewed from Earth as its apparent magnitude. But they have extended Hipparchus's system to describe the actual brightness of stars, for which they use the term absolute magnitude. For technical reasons, they define a star's absolute magnitude as what its apparent magnitude would be if it were 32.6 light-years from Earth.
Astronomers have also extended the system of magnitude numbers to include stars brighter than first magnitude and dimmer than sixth magnitude. A star that is brighter than first magnitude has a magnitude less than 1. For example, the apparent magnitude of Rigel is 0.12. Extremely bright stars have magnitudes less than zero -- that is, their designations are negative numbers. The brightest star in the night sky is Sirius, with an apparent magnitude of -1.46. Rigel has an absolute magnitude of -8.1. According to astronomers' present understanding of stars, no star can have an absolute magnitude much brighter than -8. At the other end of the scale, the dimmest stars detected with telescopes have apparent magnitudes up to 28. In theory, no star could have an absolute magnitude much fainter than 16.
Luminosity is the rate at which a star emits energy. The scientific term for a rate of energy emission is power, and scientists generally measure power in watts. For example, the luminosity of the sun is 400 trillion trillion watts. But astronomers do not usually measure a star's luminosity in watts. Instead, they express luminosities in terms of the luminosity of the sun. They often say, for instance, that the luminosity of Alpha Centauri A is about 1.3 times that of the sun and that Rigel is roughly 150,000 times as luminous as the sun.
Luminosity is related to absolute magnitude in a simple way. A difference of 5 on the absolute magnitude scale corresponds to a factor of 100 on the luminosity scale. Thus, a star with an absolute magnitude of 2 is 100 times as luminous as a star with an absolute magnitude of 7. A star with an absolute magnitude of -3 is 100 times as luminous as a star whose absolute magnitude is 2 and 10,000 times as luminous as a star that has an absolute magnitude of 7.
Color and temperature
If you look carefully at the stars, even without binoculars or a telescope, you will see a range of color from reddish to yellowish to bluish. For example, Betelgeuse looks reddish, Pollux -- like the sun -- is yellowish, and Rigel looks bluish.
A star's color depends on its surface temperature. Astronomers measure star temperatures in a metric unit known as the kelvin. One kelvin equals exactly 1 Celsius degree (1.8 Fahrenheit degree), but the Kelvin and Celsius scales start at different points. The Kelvin scale starts at -273.15 degrees C. Therefore, a temperature of 0 K equals -273.15 degrees C, or -459.67 degrees F. A temperature of 0 degrees C (32 degrees F) equals 273.15 K.
A spectacular explosion on the star Eta Carinae about 150 years ago produced three huge clouds of gas and dust -- two puffy lobes and a thin disk.
A spectacular explosion on the star Eta Carinae about 150 years ago produced three huge clouds of gas and dust -- two puffy lobes and a thin disk. Astronomers call Eta Carinae a luminous blue variable star because of its color and because it often becomes very bright -- as it did when the explosion occurred. Image credit: NASA
Dark red stars have surface temperatures of about 2500 K. The surface temperature of a bright red star is approximately 3500 K; that of the sun and other yellow stars, roughly 5500 K. Blue stars range from about 10,000 to 50,000 K in surface temperature.
Although a star appears to the unaided eye to have a single color, it actually emits a broad spectrum (band) of colors. You can see that starlight consists of many colors by using a prism to separate and spread the colors of the light of the sun, a yellow star. The visible spectrum includes all the colors of the rainbow. These colors range from red, produced by the photons (particles of light) with the least energy; to violet, produced by the most energetic photons.
Visible light is one of six bands of electromagnetic radiation. Ranging from the least energetic to the most energetic, they are: radio waves, infrared rays, visible light, ultraviolet rays, X rays, and gamma rays. All six bands are emitted by stars, but most individual stars do not emit all of them. The combined range of all six bands is known as the electromagnetic spectrum.
Astronomers study a star's spectrum by separating it, spreading it out, and displaying it. The display itself is also known as a spectrum. The scientists study thin gaps in the spectrum. When the spectrum is spread out from left to right, the gaps appear as vertical lines. The spectra of stars have dark absorption lines where radiation of specific energies is weak. In a few special cases in the visible spectrum, stars have bright emission lines where radiation of specific energies is especially strong.
An absorption line appears when a chemical element or compound absorbs radiation that has the amount of energy corresponding to the line. For example, the spectrum of the visible light coming from the sun has a group of absorption lines in the green part of the spectrum. Calcium in an outer layer of the sun absorbs light rays that would have produced the corresponding green colors.
Although all stars have absorption lines in the visible band of the electromagnetic spectrum, emission lines are more common in other parts of the spectrum. For instance, nitrogen in the sun's atmosphere emits powerful radiation that produces emission lines in the ultraviolet part of the spectrum.
Size
Astronomers measure the size of stars in terms of the sun's radius. Alpha Centauri A, with a radius of 1.05 solar radii (the plural of radius), is almost exactly the same size as the sun. Rigel is much larger at 78 solar radii, and Antares has a huge size of 776 solar radii.
A star's size and surface temperature determine its luminosity. Suppose two stars had the same temperature, but the first star had twice the radius of the second star. In this case, the first star would be four times as bright as the second star. Scientists say that luminosity is proportional to radius squared -- that is, multiplied by itself. Imagine that you wanted to compare the luminosities of two stars that had the same temperature but different radii. First, you would divide the radius of the larger star by the radius of the smaller star. Then, you would square your answer.
Now, suppose two stars had the same radius but the first star's surface temperature -- measured in kelvins -- was twice that of the second star. In this example, the luminosity of the first star would be 16 times that of the second star. Luminosity is proportional to temperature to the fourth power. Imagine that you wanted to compare the luminosities of stars that had the same radius but different temperatures. First, you would divide the temperature of the warmer star by the temperature of the cooler star. Next, you would square the result. Then, you would square your answer again.
Mass
Astronomers express the mass of a star in terms of the solar mass, the mass of the sun. For example, they give the mass of Alpha Centauri A as 1.08 solar masses; that of Rigel, as 3.50 solar masses. The mass of the sun is 2 Ž 1030 kilograms, which would be written out as 2 followed by 30 zeros.
Stars that have similar masses may not be similar in size -- that is, they may have different densities. Density is the amount of mass per unit of volume. For instance, the average density of the sun is 88 pounds per cubic foot (1,400 kilograms per cubic meter), about 140 percent that of water. Sirius B has almost exactly the same mass as the sun, but it is 90,000 times as dense. As a result, its radius is only about 1/50 of a solar radius.
The Hertzsprung-Russell diagram displays the main characteristics of stars. The diagram is named for astronomers Ejnar Hertzsprung of Denmark and Henry Norris Russell of the United States. Working independently of each other, the two scientists developed the diagram around 1910.
Luminosity classes
Points representing the brightest stars appear toward the top of the H-R diagram; points corresponding to the dimmest stars, toward the bottom. These points appear in groups that correspond to different kinds of stars. In the 1930's, American astronomers William W. Morgan and Philip C. Keenan invented what came to be known as the MK luminosity classification system for these groups. Astronomers revised and extended this system in 1978. In the MK system, the largest and brightest classes have the lowest classification numbers. The MK classes are: Ia, bright supergiant; Ib, supergiant; II, bright giant; III, giant; IV, subgiant; and V, main sequence or dwarf.
Because temperature also affects the luminosity of a star, stars from different luminosity classes can overlap. For example, Spica, a class V star, has an absolute magnitude of -3.2; but Pollux, a class III star, is dimmer, with an absolute magnitude of 0.7.
Spectral classes
Points representing the stars with the highest surface temperatures appear toward the left edge of the H-R diagram; points representing the coolest stars, toward the right edge. In the MK system, there are eight spectral classes, each corresponding to a certain range of surface temperature. From the hottest stars to the coolest, these classes are: O, B, A, F, G, K, M, and L. Each spectral class, in turn, is made up of 10 spectral types, which are designated by the letter for the spectral class and a numeral. The hottest stars in a spectral class are assigned the numeral 0; the coolest stars, the numeral 9.
A complete MK designation thus includes symbols for luminosity class and spectral type. For example, the complete designation for the sun is G2V. Alpha Centauri A is also a G2V star, and Rigel's designation is B8Ia.
Fusion in stars
A star's tremendous energy comes from a process known as nuclear fusion. This process begins when the temperature of the core of the developing star reaches about 1 million K.
A star develops from a giant, slowly rotating cloud that consists almost entirely of the chemical elements hydrogen and helium. The cloud also contains atoms of other elements as well as microscopic particles of dust.
Due to the force of its own gravity, the cloud begins to collapse inward, thereby becoming smaller. As the cloud shrinks, it rotates more and more rapidly, just as spinning ice skaters turn more rapidly when they pull in their arms. The outermost parts of the cloud form a spinning disk. The inner parts become a roughly spherical clump, which continues to collapse.
The collapsing material becomes warmer, and its pressure increases. But the pressure tends to counteract the gravitational force that is responsible for the collapse. Eventually, therefore, the collapse slows to a gradual contraction. The inner parts of the clump form a protostar, a ball-shaped object that is no longer a cloud, but is not yet a star. Surrounding the protostar is an irregular sphere of gas and dust that had been the outer parts of the clump.
Combining nuclei
When the temperature and pressure in the protostar's core become high enough, nuclear fusion begins. Nuclear fusion is a joining of two atomic nuclei to produce a larger nucleus.
Nuclei that fuse are actually the cores of atoms. A complete atom has an outer shell of one or more particles called electrons, which carry a negative electric charge. Deep inside the atom is the nucleus, which contains almost all the atom's mass. The simplest nucleus, that of the most common form of hydrogen, consists of a single particle known as a proton. A proton carries a positive electric charge. All other nuclei have one or more protons and one or more neutrons. A neutron carries no net charge, and so a nucleus is electrically positive. But a complete atom has as many electrons as protons. The net electric charge of a complete atom is therefore zero -- the atom is electrically neutral.
However, under the enormous temperatures and pressures near the core of a protostar, atoms lose electrons. The resulting atoms are known as ions, and the mixture of the free electrons and ions is called a plasma.
Atoms in the core of the protostar lose all their electrons, and the resulting bare nuclei approach one another at tremendous speeds. Under ordinary circumstances, objects that carry like charges repel each other. However, if the core temperature and pressure become high enough, the repulsion between nuclei can be overcome and the nuclei can fuse. Scientists commonly refer to fusion as "nuclear burning." But fusion has nothing to do with ordinary burning or combustion.
Converting mass to energy
When two relatively light nuclei fuse, a small amount of their mass turns into energy. Thus, the new nucleus has slightly less mass than the sum of the masses of the original nuclei. The German-born American physicist Albert Einstein discovered the relationship E = mc-squared (E=mc 2) that indicates how much energy is released when fusion occurs. The symbol E represents the energy; m, the mass that is converted; and c-squared (c2), the speed of light squared.
The speed of light is 186,282 miles (299,792 kilometers) per second. This is such a large number that the conversion of a tiny quantity of mass produces a tremendous amount of energy. For example, complete conversion of 1 gram of mass releases 90 trillion joules of energy. This amount of energy is roughly equal to the quantity released in the explosion of 22,000 tons (20,000 metric tons) of TNT. This is much more energy than was released by the atomic bomb that the United States dropped on Hiroshima, Japan, in 1945 during World War II. The energy of the bomb was equivalent to the explosion of 13,000 tons (12,000 metric tons) of TNT.
Destruction of light nuclei
In the core of a protostar, fusion begins when the temperature reaches about 1 million K. This initial fusion destroys nuclei of certain light elements. These include lithium 7 nuclei, which consist of three protons and four neutrons. In the process involving lithium 7, a hydrogen nucleus combines with a lithium 7 nucleus, which then splits into two parts. Each part consists of a nucleus of helium 4 -- two protons and two neutrons. A helium 4 nucleus is also known as an alpha particle.
Hydrogen fusion
After the light nuclei are destroyed, the protostar continues to contract. Eventually, the core temperature reaches about 10 million K, and hydrogen fusion begins. The protostar is now a star.
In hydrogen fusion, four hydrogen nuclei fuse to form a helium 4 nucleus. There are two general forms of this reaction: (1) the proton-proton (p-p) reaction and (2) the carbon-nitrogen-oxygen (CNO) cycle.
The p-p reaction can occur in several ways, including the following four-step process:
(1) Two protons fuse. In this step, two protons collide, and then one of the protons loses its positive charge by emitting a positron. The proton also emits an electrically neutral particle called a neutrino.
A positron is the antimatter equivalent of an electron. It has the same mass as an electron but differs from the electron in having a positive charge. By emitting the positron, the proton becomes a neutron. The new nucleus therefore consists of a proton and a neutron -- a combination known as a deuteron.
(2) The positron collides with an electron that happens to be nearby. As a result, the two particles annihilate each other, producing two gamma rays.
(3) The deuteron fuses with another proton, producing a helium 3 nucleus, which consists of two protons and one neutron. This step also produces a gamma ray.
(4) The helium 3 nucleus fuses with another helium 3 nucleus. This step produces a helium 4 nucleus, and two protons are released.
The CNO cycle differs from the p-p reaction mainly in that it involves carbon 12 nuclei. These nuclei consist of six protons and six neutrons. During the cycle, they change into nuclei of nitrogen 15 (7 protons and 8 neutrons) and oxygen 15 (8 protons and 7 neutrons). But they change back to carbon 12 nuclei by the end of the cycle.
Fusion of other elements
Helium nuclei can fuse to form carbon 12 nuclei. However, the core temperature must rise to about 100 million K for this process to occur. This high temperature is necessary because the helium nuclei must overcome a much higher repulsive force than the force between two protons. Each helium nucleus has two protons, so the repulsive force is four times as high as the force between two protons.
The fusion of helium is called the triple-alpha process because it combines three alpha particles to create a carbon 12 nucleus. Helium fusion also produces nuclei of oxygen 16 (8 protons and 8 neutrons) and neon 20 (10 protons and 10 neutrons).
At core temperatures of about 600 million K, carbon 12 can fuse to form sodium 23 (11 protons, 12 neutrons), magnesium 24 (12 protons, 12 neutrons), and more neon 20. However, not all stars can reach these temperatures.
As fusion processes produce heavier and heavier elements, the temperature necessary for further processes increases. At about 1 billion K, oxygen 16 nuclei can fuse, producing silicon 28 (14 protons, 14 neutrons), phosphorus 31 (15 protons, 16 neutrons), and sulfur 32 (16 protons, 16 neutrons).
Fusion can produce energy only as long as the new nuclei have less mass than the sum of the masses of the original nuclei. Energy production continues until nuclei of iron 56 (26 protons, 30 neutrons) begin to combine with other nuclei. When this happens, the new nuclei have slightly more mass than the original nuclei. This process therefore uses energy, rather than producing it.
Evolution of stars
The life cycles of stars follow three general patterns, each associated with a range of initial mass. There are (1) high-mass stars, which have more than 8 solar masses; (2) intermediate-mass stars, with 0.5 to 8 solar masses -- the group that includes the sun; and (3) low-mass stars, with 0.1 to 0.5 solar mass. Objects with less than 0.1 solar mass do not have enough gravitational force to produce the core temperature necessary for hydrogen fusion.
The life cycles of single stars are simpler than those of binary systems, so this section discusses the evolution of single stars first. And because astronomers know much more about the sun than any other star, the discussion begins with the development of intermediate-mass stars.
Intermediate-mass stars
A cloud that eventually develops into an intermediate-mass star takes about 100,000 years to collapse into a protostar. As a protostar, it has a surface temperature of about 4000 K. It may be anywhere from a few times to a few thousand times as luminous as the sun, depending on its mass.
T-Tauri phase
When hydrogen fusion begins, the protostar is still surrounded by an irregular mass of gas and dust. But the energy produced by hydrogen fusion pushes away this material as a protostellar wind. In many cases, the disk that is left over from the collapse channels the wind into two narrow cones or jets. One jet emerges from each side of the disk at a right angle to the plane of the disk. The protostar has become a T-Tauri star, a type of object named after the star T in the constellation Taurus (the Bull). A T-Tauri star is a variable star, one that varies in brightness.
Main-sequence phase
The T-Tauri star contracts for about 10 million years. It stops contracting when its tendency to expand due to the energy produced by fusion in its core balances its tendency to contract due to gravity. By this time, hydrogen fusion in the core is supplying all the star's energy. The star has begun the longest part of its life as a producer of energy from hydrogen fusion, the main-sequence phase. The name of this phase comes from a part of the H-R diagram.
Any star -- whatever its mass -- that gets all its energy from hydrogen fusion in its core is said to be "on the main sequence" or "a main-sequence star." The amount of time a star spends there depends on its mass. The greater a star's mass, the more rapidly the hydrogen in its core is used up, and therefore the shorter is its stay on the main sequence. An intermediate-mass star remains on the main sequence for billions of years.
Red giant phase
When all the hydrogen in the core of an intermediate-mass star has fused into helium, the star changes rapidly. Because the core no longer produces fusion energy, gravity immediately crushes matter down upon it. The resulting compression quickly heats the core and the region around it. The temperature becomes so high that hydrogen fusion begins in a thin shell surrounding the core. This fusion produces even more energy than had been produced by hydrogen fusion in the core. The extra energy pushes against the star's outer layers, and so the star expands enormously.
As the star expands, its outer layers become cooler, so the star becomes redder. And because the star's surface area expands greatly, the star also becomes brighter. The star is now a red giant.
Horizontal branch phase
Eventually, the core temperature reaches 100 million K, high enough to support the triple-alpha process. This process begins so rapidly that its onset is known as helium flash.
As the triple-alpha process continues, the core expands, but its temperature drops. This decrease in temperature causes the temperature of the hydrogen-burning shell to drop. Consequently, the energy output of the shell decreases, and the outer layers of the star contract. The star becomes hotter but smaller and fainter than it had been as a red giant. This change occurs over a period of about 100 million years.
At the end of this period, the star is in its horizontal branch phase, named for the position of the point representing the star on the H-R diagram. The star steadily burns helium and hydrogen, and so its temperature, size, and luminosity do not change significantly. This phase lasts for about 10 million years.
Asymptotic giant phase
When all the helium in the core has fused, the core contracts and therefore becomes hotter. The triple-alpha process begins in a shell surrounding the core, and hydrogen fusion continues in a shell surrounding that. Due to the increased energy produced by the burning in the shells, the star's outer layers expand. The star becomes a giant again, but it is bluer and brighter than it was the first time.
On the H-R diagram, the point representing the star has moved upward and to the right along a line known as the asymptotic (as ihm TOT ihk) giant branch (AGB). The star is therefore called an AGB star.
An AGB star's core is so hot and its gravitational grip on its outermost layers is so weak that those layers blow away in a stellar wind. As each layer blows away, a hotter layer is exposed. Thus, the stellar wind becomes even stronger. Out in space, a succession of new, fast winds slam into old, slow winds that are still moving away from the star. The collisions produce dense shells of gas, some of which cool to form dust.
White dwarf phase
A planetary nebula with an unusual textured appearance, the cause of which is unknown. This photo was taken by the Hubble Space Telescope.
A planetary nebula with an unusual textured appearance, the cause of which is unknown. This photo was taken by the Hubble Space Telescope. Image credit: NASA
In just a few thousand years, all but the hot core of an AGB star blows away, and fusion ceases in the core. The core illuminates the surrounding shells. Such shells looked like planets through the crude telescopes of astronomers who studied them in the 1800's. As a result, the astronomers called the shells planetary nebulae -- and today's astronomers still do. The word nebulae is Latin for clouds.
After a planetary nebula fades from view, the remaining core is known as a white dwarf star. This kind of star consists mostly of carbon and oxygen. Its initial temperature is about 100,000 K.
Black dwarf phase
Because a white dwarf star has no fuel remaining for fusion, it becomes cooler and cooler. Over billions of years, it cools more and more slowly. Eventually, it becomes a black dwarf -- an object too faint to detect. A black dwarf represents the end of the life cycle of an intermediate-mass star.
High-mass stars, those with more than 8 solar masses, form quickly and have short lives. A high-mass star forms from a protostar in about 10,000 to 100,000 years.
High-mass stars on the main sequence are hot and blue. They are 1,000 to 1 million times as luminous as the sun, and their radii are about 10 times the solar radius. High-mass stars are much less common than intermediate- and low-mass stars. Because they are so bright, however, high-mass stars are visible from great distances, and so many are known.
A high-mass star has a strong stellar wind. A star of 30 solar masses can lose 24 solar masses by stellar wind before its core runs out of hydrogen and it leaves the main sequence.
As a high-mass star leaves the main sequence, hydrogen begins to fuse in a shell outside its core. As a result, its radius increases to about 100 times that of the sun. However, its luminosity decreases slightly. Because the star is now emitting almost the same amount of energy from a much larger surface, the temperature of the surface decreases. The star therefore becomes redder.
As the star evolves, its core heats up to 100 million K, enough to start the triple-alpha process. After about 1 million years, helium fusion ends in the core but begins in a shell outside the core. And, as in an intermediate-mass star, hydrogen fuses in a shell outside that. The high-mass star becomes a bright red supergiant.
When the contracting core becomes sufficiently hot, carbon fuses, producing neon, sodium, and magnesium. This phase lasts only about 10,000 years. A succession of fusion processes then occur in the core. Each successive process involves a different element and takes less time. Whenever a different element begins to fuse in the core, the element that had been fusing there continues to fuse in a shell outside the core. In addition, all the elements that had been fusing in shells continue to do so. Neon fuses to produce oxygen and magnesium, a process that lasts about 12 years. Oxygen then fuses, producing silicon and sulfur for about 4 years. Finally, silicon fuses to make iron, taking about a week.
Supernovae
At this time, the radius of the iron core is about 1,900 miles (3,000 kilometers). Because further fusion would consume energy, the star is now doomed. It cannot produce any more fusion energy to balance the force of gravity.
When the mass of the iron core reaches 1.4 solar masses, violent events occur. The force of gravity within the core causes the core to collapse. As a result, the core temperature rises to nearly 10 billion K. At this temperature, the iron nuclei break down into lighter nuclei and eventually into individual protons and neutrons. As the collapse continues, protons combine with electrons, producing neutrons and neutrinos. The neutrinos carry away about 99 percent of the energy produced by the crushing of the core.
Now, the core consists of a collapsing ball of neutrons. When the radius of the ball shrinks to about 6 miles (10 kilometers), the ball rebounds like a solid rubber ball that has been squeezed.
All the events from the beginning of the collapse of the core to the rebounding of the neutrons occur in about one second. But more violence is in store. The rebounding of the ball of neutrons sends a spherical shock wave outward through the star. Much of the energy of the wave causes fusion to occur in overlying layers, creating new elements. As the wave reaches the star's surface, it boosts temperatures to 200,000 K. As a result, the star explodes, hurling matter into space at speeds of about 9,000 to 25,000 miles (15,000 to 40,000 kilometers) per second. The brilliant explosion is known as a Type II supernova.
Supernovae enrich the clouds of gas and dust from which new stars eventually form. This enrichment process has been going on since the first supernovae billions of years ago. Supernovae in the first generation of stars enriched the clouds with materials that later went into making newer stars.
Three generations of stars may exist. Astronomers have not found any of what would be the oldest generation, Population III, stars. But they have found members of the other two generations. Population II stars, which would be the second generation, contain relatively small amounts of heavy elements. The more massive ones aged and died quickly, thereby contributing more nuclei of heavy elements to the clouds. For this reason, Population I stars, the third generation, contain the largest amounts of heavy elements. Yet these quantities are tiny compared with the amount of hydrogen and helium in Population I stars. For example, elements other than hydrogen and helium make up from 1 to 2 percent of the mass of the sun, a Population I star.
Neutron stars
After a Type II supernova blast occurs, the stellar core remains behind. If the core has less than about 3 solar masses, it becomes a neutron star. This object consists almost entirely of neutrons. It packs at least 1.4 solar masses into a sphere with a radius of about 6 to 10 miles (10 to 15 kilometers).
Neutron stars have initial temperatures of 10 million K, but they are so small that their visible light is difficult to detect. However, astronomers have detected pulses of radio energy from neutron stars, sometimes at a rate of almost 1,000 pulses per second.
A neutron star actually emits two continuous beams of radio energy. The beams flow away from the star in opposite directions. As the star rotates, the beams sweep around in space like searchlight beams. If one of the beams periodically sweeps over Earth, a radio telescope can detect it as a series of pulses. The telescope detects one pulse for each revolution of the star. A star that is detected in this way is known as a pulsar.
Black holes
If the stellar core remaining after the supernova explosion has about 3 or more solar masses, no known force can support it against its own gravitation. The core collapses to form a black hole, a region of space whose gravitational force is so strong that nothing can escape from it. A black hole is invisible because it traps even light. All its matter is located at a single point in its center. This point, known as a singularity, is much smaller than an atomic nucleus.
Low-mass stars, ranging from 0.1 to 0.5 solar mass, have surface temperatures less than about 4,000 K. Their luminosities are less than 2 percent of the solar luminosity. Low-mass stars use hydrogen fuel so slowly that they may shine as main-sequence stars for 100 billion to 1 trillion years. This life span is longer than the present age of the universe, believed to be 10 billion to 20 billion years. Therefore, no low-mass star has ever died. Nevertheless, astronomers have determined that low-mass stars will never fuse anything but hydrogen. Thus, as these stars die, they will not pass through a red-giant phase. Instead, they will merely cool to become white dwarfs, then black dwarfs.
Binary stars develop from two protostars that form near each other. More than 50 percent of what seem to the unaided eye to be single stars are actually binaries.
Transfer of mass occurs in a binary star system. Matter flows from a sunlike star, in the background in this illustration, to a disk orbiting a white dwarf star, then to the surface of the dwarf.
Transfer of mass occurs in a binary star system. Matter flows from a sunlike star, in the background in this illustration, to a disk orbiting a white dwarf star, then to the surface of the dwarf. Image credit: Space Telescope Science Institute
One star in a binary system can affect the life cycle of the other if the two stars are sufficiently close together. Between the stars is a location called the Lagrange point, named for the French mathematician Joseph Louis Lagrange, where the star's gravitational forces are exactly equal. If one of the stars expands so much that its outer layers pass the Lagrange point, the other star will begin to strip away those layers and accumulate them on its surface.
This process, called mass transfer, can take many forms. Mass transfer from a red giant onto a main-sequence companion can add absorption lines of carbon or other elements to the spectrum of the main- sequence star. But if the stars are close together, the material will flow in the opposite direction when the giant star becomes a white dwarf. The matter will spiral in toward the dwarf, forming a hot disk around it. The disk will flare brilliantly in visible and ultraviolet radiation.
If the giant star leaves behind a neutron star or a black hole instead of a white dwarf, an X-ray binary may form. In this case, the matter transferred from the main-sequence star will become extremely hot. When this matter strikes the surface of the neutron star or is pulled into the black hole, it will emit X rays.
In a third case, the red giant becomes a white dwarf, and the main-sequence star becomes a red giant. When enough gas from the giant accumulates on the dwarf's surface, gas nuclei will fuse violently in a flash called a nova. In some cases, so much gas will accumulate that its weight will cause the dwarf to collapse. Almost instantly, the dwarf's carbon will fuse, and the entire dwarf will explode in a Type I supernova. This kind of explosion is so bright that it can outshine an entire galaxy for a few months.
Contributor: Paul J. Green, Ph.D., Astrophysicist, Smithsonian Astrophysical Observatory.
A globular cluster is a tightly grouped swarm of stars held together by gravity. This globular cluster is one of the densest of the 147 known clusters in the Milky Way galaxy.
A globular cluster is a tightly grouped swarm of stars held together by gravity. This globular cluster is one of the densest of the 147 known clusters in the Milky Way galaxy. Image credit: NASA
A star is a huge, shining ball in space that produces a tremendous amount of light and other forms of energy. The sun is a star, and it supplies Earth with light and heat energy. The stars look like twinkling points of light -- except for the sun. The sun looks like a ball because it is much closer to Earth than any other star.
The sun and most other stars are made of gas and a hot, gaslike substance known as plasma. But some stars, called white dwarfs and neutron stars, consist of tightly packed atoms or subatomic particles. These stars are therefore much more dense than anything on Earth.
Stars come in many sizes. The sun's radius (distance from its center to its surface) is about 432,000 miles (695,500 kilometers). But astronomers classify the sun as a dwarf because other kinds of stars are much bigger. Some of the stars known as supergiants have a radius about 1,000 times that of the sun. The smallest stars are the neutron stars, some of which have a radius of only about 6 miles (10 kilometers).
About 75 percent of all stars are members of a binary system, a pair of closely spaced stars that orbit each other. The sun is not a member of a binary system. However, its nearest known stellar neighbor, Proxima Centauri, is part of a multiple-star system that also includes Alpha Centauri A and Alpha Centauri B.
The distance from the sun to Proxima Centauri is more than 25 trillion miles (40 trillion kilometers). This distance is so great that light takes 4.2 years to travel between the two stars. Scientists say that Proxima Centauri is 4.2 light-years from the sun. One light-year, the distance that light travels in a vacuum in a year, equals about 5.88 trillion miles (9.46 trillion kilometers).
Stars are grouped in huge structures called galaxies. Telescopes have revealed galaxies throughout the universe at distances of 12 billion to 16 billion light-years. The sun is in a galaxy called the Milky Way that contains more than 100 billion stars. There are more than 100 billion galaxies in the universe, and the average number of stars per galaxy may be 100 billion. Thus, more than 10 billion trillion stars may exist. But if you look at the night sky far from city lights, you can see only about 3,000 of them without using binoculars or a telescope.
Stars, like people, have life cycles -- they are born, pass through several phases, and eventually die. The sun was born about 4.6 billion years ago and will remain much as it is for another 5 billion years. Then it will grow to become a red giant. Late in the sun's lifetime, it will cast off its outer layers. The remaining core, called a white dwarf, will slowly fade to become a black dwarf.
Other stars will end their lives in different ways. Some will not go through a red giant stage. Instead, they will merely cool to become white dwarfs, then black dwarfs. A small percentage of stars will die in spectacular explosions called supernovae.
This article discusses Star (The stars at night) (Names of stars) (Characteristics of stars) (Fusion in stars) (Evolution of stars).
The stars at night
If you look at the stars on a clear night, you will notice that they seem to twinkle and that they differ greatly in brightness. A much slower movement also takes place in the night sky: If you map the location of several stars for a few hours, you will observe that all the stars revolve slowly about a single point in the sky.
Twinkling of stars is caused by movements in Earth's atmosphere. Starlight enters the atmosphere as straight rays. Twinkling occurs because air movements constantly change the path of the light as it comes through the air. You can see a similar effect if you stand in a swimming pool and look down. Unless the water is almost perfectly still, your feet will appear to move and change their shape. This "twinkling" occurs because the moving water constantly changes the path of the light rays that travel from your feet to your eyes.
Brightness of stars. How bright a star looks when viewed from Earth depends on two factors: (1) the actual brightness of the star -- that is, the amount of light energy the star emits (sends out) -- and (2) the distance from Earth to the star. A nearby star that is actually dim can appear brighter than a distant star that is really extremely brilliant. For example, Alpha Centauri A seems to be slightly brighter than a star known as Rigel. But Alpha Centauri A emits only 1/100,000 as much light energy as Rigel. Alpha Centauri A seems brighter because it is only 1/325 as far from Earth as Rigel is -- 4.4 light-years for Alpha Centauri A, 1,400 light-years for Rigel.
Rising and setting of stars
When viewed from Earth's Northern Hemisphere, stars rotate counterclockwise around a point called the celestial north pole. Viewed from the Southern Hemisphere, stars rotate clockwise about the celestial south pole. During the day, the sun moves across the sky in the same direction, and at the same rate, as the stars. These movements do not result from any actual revolution of the sun and stars. Rather, they occur because of the west-to-east rotation of Earth about its own axis. To an observer standing on the ground, Earth seems motionless, while the sun and stars seem to move in circles. But actually, Earth moves.
Names of stars
Ancient people saw that certain stars are arranged in patterns shaped somewhat like human beings, animals, or common objects. Some of these patterns, called constellations, came to represent figures of mythological characters. For example, the constellation Orion (the Hunter) is named after a hero in Greek mythology.
Today, astronomers use constellations, some of which were described by the ancients, in the scientific names of stars. The International Astronomical Union (IAU), the world authority for assigning names to celestial objects, officially recognizes 88 constellations. These constellations cover the entire sky. In most cases, the brightest star in a given constellation has alpha -- the first letter of the Greek alphabet -- as part of its scientific name. For instance, the scientific name for Vega, the brightest star in the constellation Lyra (the Harp), is Alpha Lyrae. Lyrae is Latin for of Lyra.
The second brightest star in a constellation is usually designated beta, the second letter of the Greek alphabet, the third brightest is gamma, and so on. The assignment of Greek letters to stars continues until all the Greek letters are used. Numerical designations follow.
But the number of known stars has become so large that the IAU uses a different system for newly discovered stars. Most new names consist of an abbreviation followed by a group of symbols. The abbreviation stands for either the type of star or a catalog that lists information about the star. For example, PSR J1302-6350 is a type of star known as a pulsar -- hence the PSR in its name. The symbols indicate the star's location in the sky. The 1302 and the 6350 are coordinates that are similar to the longitude and latitude designations used to indicate locations on Earth's surface. The J indicates that a coordinate system known as J2000 is being used.
Characteristics of stars
A star has five main characteristics: (1) brightness, which astronomers describe in terms of magnitude or luminosity; (2) color; (3) surface temperature; (4) size; and (5) mass (amount of matter). These characteristics are related to one another in a complex way. Color depends on surface temperature, and brightness depends on surface temperature and size. Mass affects the rate at which a star of a given size produces energy and so affects surface temperature. To make these relationships easier to understand, astronomers developed a graph called the Hertzsprung-Russell (H-R) diagram. This graph, a version of which appears in this article, also helps astronomers understand and describe the life cycles of stars.
Magnitude and luminosity
Magnitude is based on a numbering system invented by the Greek astronomer Hipparchus in about 125 B.C. Hipparchus numbered groups of stars according to their brightness as viewed from Earth. He called the brightest stars first magnitude stars, the next brightest second magnitude stars, and so on to sixth magnitude stars, the faintest visible stars.
Modern astronomers refer to a star's brightness as viewed from Earth as its apparent magnitude. But they have extended Hipparchus's system to describe the actual brightness of stars, for which they use the term absolute magnitude. For technical reasons, they define a star's absolute magnitude as what its apparent magnitude would be if it were 32.6 light-years from Earth.
Astronomers have also extended the system of magnitude numbers to include stars brighter than first magnitude and dimmer than sixth magnitude. A star that is brighter than first magnitude has a magnitude less than 1. For example, the apparent magnitude of Rigel is 0.12. Extremely bright stars have magnitudes less than zero -- that is, their designations are negative numbers. The brightest star in the night sky is Sirius, with an apparent magnitude of -1.46. Rigel has an absolute magnitude of -8.1. According to astronomers' present understanding of stars, no star can have an absolute magnitude much brighter than -8. At the other end of the scale, the dimmest stars detected with telescopes have apparent magnitudes up to 28. In theory, no star could have an absolute magnitude much fainter than 16.
Luminosity is the rate at which a star emits energy. The scientific term for a rate of energy emission is power, and scientists generally measure power in watts. For example, the luminosity of the sun is 400 trillion trillion watts. But astronomers do not usually measure a star's luminosity in watts. Instead, they express luminosities in terms of the luminosity of the sun. They often say, for instance, that the luminosity of Alpha Centauri A is about 1.3 times that of the sun and that Rigel is roughly 150,000 times as luminous as the sun.
Luminosity is related to absolute magnitude in a simple way. A difference of 5 on the absolute magnitude scale corresponds to a factor of 100 on the luminosity scale. Thus, a star with an absolute magnitude of 2 is 100 times as luminous as a star with an absolute magnitude of 7. A star with an absolute magnitude of -3 is 100 times as luminous as a star whose absolute magnitude is 2 and 10,000 times as luminous as a star that has an absolute magnitude of 7.
Color and temperature
If you look carefully at the stars, even without binoculars or a telescope, you will see a range of color from reddish to yellowish to bluish. For example, Betelgeuse looks reddish, Pollux -- like the sun -- is yellowish, and Rigel looks bluish.
A star's color depends on its surface temperature. Astronomers measure star temperatures in a metric unit known as the kelvin. One kelvin equals exactly 1 Celsius degree (1.8 Fahrenheit degree), but the Kelvin and Celsius scales start at different points. The Kelvin scale starts at -273.15 degrees C. Therefore, a temperature of 0 K equals -273.15 degrees C, or -459.67 degrees F. A temperature of 0 degrees C (32 degrees F) equals 273.15 K.
A spectacular explosion on the star Eta Carinae about 150 years ago produced three huge clouds of gas and dust -- two puffy lobes and a thin disk.
A spectacular explosion on the star Eta Carinae about 150 years ago produced three huge clouds of gas and dust -- two puffy lobes and a thin disk. Astronomers call Eta Carinae a luminous blue variable star because of its color and because it often becomes very bright -- as it did when the explosion occurred. Image credit: NASA
Dark red stars have surface temperatures of about 2500 K. The surface temperature of a bright red star is approximately 3500 K; that of the sun and other yellow stars, roughly 5500 K. Blue stars range from about 10,000 to 50,000 K in surface temperature.
Although a star appears to the unaided eye to have a single color, it actually emits a broad spectrum (band) of colors. You can see that starlight consists of many colors by using a prism to separate and spread the colors of the light of the sun, a yellow star. The visible spectrum includes all the colors of the rainbow. These colors range from red, produced by the photons (particles of light) with the least energy; to violet, produced by the most energetic photons.
Visible light is one of six bands of electromagnetic radiation. Ranging from the least energetic to the most energetic, they are: radio waves, infrared rays, visible light, ultraviolet rays, X rays, and gamma rays. All six bands are emitted by stars, but most individual stars do not emit all of them. The combined range of all six bands is known as the electromagnetic spectrum.
Astronomers study a star's spectrum by separating it, spreading it out, and displaying it. The display itself is also known as a spectrum. The scientists study thin gaps in the spectrum. When the spectrum is spread out from left to right, the gaps appear as vertical lines. The spectra of stars have dark absorption lines where radiation of specific energies is weak. In a few special cases in the visible spectrum, stars have bright emission lines where radiation of specific energies is especially strong.
An absorption line appears when a chemical element or compound absorbs radiation that has the amount of energy corresponding to the line. For example, the spectrum of the visible light coming from the sun has a group of absorption lines in the green part of the spectrum. Calcium in an outer layer of the sun absorbs light rays that would have produced the corresponding green colors.
Although all stars have absorption lines in the visible band of the electromagnetic spectrum, emission lines are more common in other parts of the spectrum. For instance, nitrogen in the sun's atmosphere emits powerful radiation that produces emission lines in the ultraviolet part of the spectrum.
Size
Astronomers measure the size of stars in terms of the sun's radius. Alpha Centauri A, with a radius of 1.05 solar radii (the plural of radius), is almost exactly the same size as the sun. Rigel is much larger at 78 solar radii, and Antares has a huge size of 776 solar radii.
A star's size and surface temperature determine its luminosity. Suppose two stars had the same temperature, but the first star had twice the radius of the second star. In this case, the first star would be four times as bright as the second star. Scientists say that luminosity is proportional to radius squared -- that is, multiplied by itself. Imagine that you wanted to compare the luminosities of two stars that had the same temperature but different radii. First, you would divide the radius of the larger star by the radius of the smaller star. Then, you would square your answer.
Now, suppose two stars had the same radius but the first star's surface temperature -- measured in kelvins -- was twice that of the second star. In this example, the luminosity of the first star would be 16 times that of the second star. Luminosity is proportional to temperature to the fourth power. Imagine that you wanted to compare the luminosities of stars that had the same radius but different temperatures. First, you would divide the temperature of the warmer star by the temperature of the cooler star. Next, you would square the result. Then, you would square your answer again.
Mass
Astronomers express the mass of a star in terms of the solar mass, the mass of the sun. For example, they give the mass of Alpha Centauri A as 1.08 solar masses; that of Rigel, as 3.50 solar masses. The mass of the sun is 2 Ž 1030 kilograms, which would be written out as 2 followed by 30 zeros.
Stars that have similar masses may not be similar in size -- that is, they may have different densities. Density is the amount of mass per unit of volume. For instance, the average density of the sun is 88 pounds per cubic foot (1,400 kilograms per cubic meter), about 140 percent that of water. Sirius B has almost exactly the same mass as the sun, but it is 90,000 times as dense. As a result, its radius is only about 1/50 of a solar radius.
The Hertzsprung-Russell diagram displays the main characteristics of stars. The diagram is named for astronomers Ejnar Hertzsprung of Denmark and Henry Norris Russell of the United States. Working independently of each other, the two scientists developed the diagram around 1910.
Luminosity classes
Points representing the brightest stars appear toward the top of the H-R diagram; points corresponding to the dimmest stars, toward the bottom. These points appear in groups that correspond to different kinds of stars. In the 1930's, American astronomers William W. Morgan and Philip C. Keenan invented what came to be known as the MK luminosity classification system for these groups. Astronomers revised and extended this system in 1978. In the MK system, the largest and brightest classes have the lowest classification numbers. The MK classes are: Ia, bright supergiant; Ib, supergiant; II, bright giant; III, giant; IV, subgiant; and V, main sequence or dwarf.
Because temperature also affects the luminosity of a star, stars from different luminosity classes can overlap. For example, Spica, a class V star, has an absolute magnitude of -3.2; but Pollux, a class III star, is dimmer, with an absolute magnitude of 0.7.
Spectral classes
Points representing the stars with the highest surface temperatures appear toward the left edge of the H-R diagram; points representing the coolest stars, toward the right edge. In the MK system, there are eight spectral classes, each corresponding to a certain range of surface temperature. From the hottest stars to the coolest, these classes are: O, B, A, F, G, K, M, and L. Each spectral class, in turn, is made up of 10 spectral types, which are designated by the letter for the spectral class and a numeral. The hottest stars in a spectral class are assigned the numeral 0; the coolest stars, the numeral 9.
A complete MK designation thus includes symbols for luminosity class and spectral type. For example, the complete designation for the sun is G2V. Alpha Centauri A is also a G2V star, and Rigel's designation is B8Ia.
Fusion in stars
A star's tremendous energy comes from a process known as nuclear fusion. This process begins when the temperature of the core of the developing star reaches about 1 million K.
A star develops from a giant, slowly rotating cloud that consists almost entirely of the chemical elements hydrogen and helium. The cloud also contains atoms of other elements as well as microscopic particles of dust.
Due to the force of its own gravity, the cloud begins to collapse inward, thereby becoming smaller. As the cloud shrinks, it rotates more and more rapidly, just as spinning ice skaters turn more rapidly when they pull in their arms. The outermost parts of the cloud form a spinning disk. The inner parts become a roughly spherical clump, which continues to collapse.
The collapsing material becomes warmer, and its pressure increases. But the pressure tends to counteract the gravitational force that is responsible for the collapse. Eventually, therefore, the collapse slows to a gradual contraction. The inner parts of the clump form a protostar, a ball-shaped object that is no longer a cloud, but is not yet a star. Surrounding the protostar is an irregular sphere of gas and dust that had been the outer parts of the clump.
Combining nuclei
When the temperature and pressure in the protostar's core become high enough, nuclear fusion begins. Nuclear fusion is a joining of two atomic nuclei to produce a larger nucleus.
Nuclei that fuse are actually the cores of atoms. A complete atom has an outer shell of one or more particles called electrons, which carry a negative electric charge. Deep inside the atom is the nucleus, which contains almost all the atom's mass. The simplest nucleus, that of the most common form of hydrogen, consists of a single particle known as a proton. A proton carries a positive electric charge. All other nuclei have one or more protons and one or more neutrons. A neutron carries no net charge, and so a nucleus is electrically positive. But a complete atom has as many electrons as protons. The net electric charge of a complete atom is therefore zero -- the atom is electrically neutral.
However, under the enormous temperatures and pressures near the core of a protostar, atoms lose electrons. The resulting atoms are known as ions, and the mixture of the free electrons and ions is called a plasma.
Atoms in the core of the protostar lose all their electrons, and the resulting bare nuclei approach one another at tremendous speeds. Under ordinary circumstances, objects that carry like charges repel each other. However, if the core temperature and pressure become high enough, the repulsion between nuclei can be overcome and the nuclei can fuse. Scientists commonly refer to fusion as "nuclear burning." But fusion has nothing to do with ordinary burning or combustion.
Converting mass to energy
When two relatively light nuclei fuse, a small amount of their mass turns into energy. Thus, the new nucleus has slightly less mass than the sum of the masses of the original nuclei. The German-born American physicist Albert Einstein discovered the relationship E = mc-squared (E=mc 2) that indicates how much energy is released when fusion occurs. The symbol E represents the energy; m, the mass that is converted; and c-squared (c2), the speed of light squared.
The speed of light is 186,282 miles (299,792 kilometers) per second. This is such a large number that the conversion of a tiny quantity of mass produces a tremendous amount of energy. For example, complete conversion of 1 gram of mass releases 90 trillion joules of energy. This amount of energy is roughly equal to the quantity released in the explosion of 22,000 tons (20,000 metric tons) of TNT. This is much more energy than was released by the atomic bomb that the United States dropped on Hiroshima, Japan, in 1945 during World War II. The energy of the bomb was equivalent to the explosion of 13,000 tons (12,000 metric tons) of TNT.
Destruction of light nuclei
In the core of a protostar, fusion begins when the temperature reaches about 1 million K. This initial fusion destroys nuclei of certain light elements. These include lithium 7 nuclei, which consist of three protons and four neutrons. In the process involving lithium 7, a hydrogen nucleus combines with a lithium 7 nucleus, which then splits into two parts. Each part consists of a nucleus of helium 4 -- two protons and two neutrons. A helium 4 nucleus is also known as an alpha particle.
Hydrogen fusion
After the light nuclei are destroyed, the protostar continues to contract. Eventually, the core temperature reaches about 10 million K, and hydrogen fusion begins. The protostar is now a star.
In hydrogen fusion, four hydrogen nuclei fuse to form a helium 4 nucleus. There are two general forms of this reaction: (1) the proton-proton (p-p) reaction and (2) the carbon-nitrogen-oxygen (CNO) cycle.
The p-p reaction can occur in several ways, including the following four-step process:
(1) Two protons fuse. In this step, two protons collide, and then one of the protons loses its positive charge by emitting a positron. The proton also emits an electrically neutral particle called a neutrino.
A positron is the antimatter equivalent of an electron. It has the same mass as an electron but differs from the electron in having a positive charge. By emitting the positron, the proton becomes a neutron. The new nucleus therefore consists of a proton and a neutron -- a combination known as a deuteron.
(2) The positron collides with an electron that happens to be nearby. As a result, the two particles annihilate each other, producing two gamma rays.
(3) The deuteron fuses with another proton, producing a helium 3 nucleus, which consists of two protons and one neutron. This step also produces a gamma ray.
(4) The helium 3 nucleus fuses with another helium 3 nucleus. This step produces a helium 4 nucleus, and two protons are released.
The CNO cycle differs from the p-p reaction mainly in that it involves carbon 12 nuclei. These nuclei consist of six protons and six neutrons. During the cycle, they change into nuclei of nitrogen 15 (7 protons and 8 neutrons) and oxygen 15 (8 protons and 7 neutrons). But they change back to carbon 12 nuclei by the end of the cycle.
Fusion of other elements
Helium nuclei can fuse to form carbon 12 nuclei. However, the core temperature must rise to about 100 million K for this process to occur. This high temperature is necessary because the helium nuclei must overcome a much higher repulsive force than the force between two protons. Each helium nucleus has two protons, so the repulsive force is four times as high as the force between two protons.
The fusion of helium is called the triple-alpha process because it combines three alpha particles to create a carbon 12 nucleus. Helium fusion also produces nuclei of oxygen 16 (8 protons and 8 neutrons) and neon 20 (10 protons and 10 neutrons).
At core temperatures of about 600 million K, carbon 12 can fuse to form sodium 23 (11 protons, 12 neutrons), magnesium 24 (12 protons, 12 neutrons), and more neon 20. However, not all stars can reach these temperatures.
As fusion processes produce heavier and heavier elements, the temperature necessary for further processes increases. At about 1 billion K, oxygen 16 nuclei can fuse, producing silicon 28 (14 protons, 14 neutrons), phosphorus 31 (15 protons, 16 neutrons), and sulfur 32 (16 protons, 16 neutrons).
Fusion can produce energy only as long as the new nuclei have less mass than the sum of the masses of the original nuclei. Energy production continues until nuclei of iron 56 (26 protons, 30 neutrons) begin to combine with other nuclei. When this happens, the new nuclei have slightly more mass than the original nuclei. This process therefore uses energy, rather than producing it.
Evolution of stars
The life cycles of stars follow three general patterns, each associated with a range of initial mass. There are (1) high-mass stars, which have more than 8 solar masses; (2) intermediate-mass stars, with 0.5 to 8 solar masses -- the group that includes the sun; and (3) low-mass stars, with 0.1 to 0.5 solar mass. Objects with less than 0.1 solar mass do not have enough gravitational force to produce the core temperature necessary for hydrogen fusion.
The life cycles of single stars are simpler than those of binary systems, so this section discusses the evolution of single stars first. And because astronomers know much more about the sun than any other star, the discussion begins with the development of intermediate-mass stars.
Intermediate-mass stars
A cloud that eventually develops into an intermediate-mass star takes about 100,000 years to collapse into a protostar. As a protostar, it has a surface temperature of about 4000 K. It may be anywhere from a few times to a few thousand times as luminous as the sun, depending on its mass.
T-Tauri phase
When hydrogen fusion begins, the protostar is still surrounded by an irregular mass of gas and dust. But the energy produced by hydrogen fusion pushes away this material as a protostellar wind. In many cases, the disk that is left over from the collapse channels the wind into two narrow cones or jets. One jet emerges from each side of the disk at a right angle to the plane of the disk. The protostar has become a T-Tauri star, a type of object named after the star T in the constellation Taurus (the Bull). A T-Tauri star is a variable star, one that varies in brightness.
Main-sequence phase
The T-Tauri star contracts for about 10 million years. It stops contracting when its tendency to expand due to the energy produced by fusion in its core balances its tendency to contract due to gravity. By this time, hydrogen fusion in the core is supplying all the star's energy. The star has begun the longest part of its life as a producer of energy from hydrogen fusion, the main-sequence phase. The name of this phase comes from a part of the H-R diagram.
Any star -- whatever its mass -- that gets all its energy from hydrogen fusion in its core is said to be "on the main sequence" or "a main-sequence star." The amount of time a star spends there depends on its mass. The greater a star's mass, the more rapidly the hydrogen in its core is used up, and therefore the shorter is its stay on the main sequence. An intermediate-mass star remains on the main sequence for billions of years.
Red giant phase
When all the hydrogen in the core of an intermediate-mass star has fused into helium, the star changes rapidly. Because the core no longer produces fusion energy, gravity immediately crushes matter down upon it. The resulting compression quickly heats the core and the region around it. The temperature becomes so high that hydrogen fusion begins in a thin shell surrounding the core. This fusion produces even more energy than had been produced by hydrogen fusion in the core. The extra energy pushes against the star's outer layers, and so the star expands enormously.
As the star expands, its outer layers become cooler, so the star becomes redder. And because the star's surface area expands greatly, the star also becomes brighter. The star is now a red giant.
Horizontal branch phase
Eventually, the core temperature reaches 100 million K, high enough to support the triple-alpha process. This process begins so rapidly that its onset is known as helium flash.
As the triple-alpha process continues, the core expands, but its temperature drops. This decrease in temperature causes the temperature of the hydrogen-burning shell to drop. Consequently, the energy output of the shell decreases, and the outer layers of the star contract. The star becomes hotter but smaller and fainter than it had been as a red giant. This change occurs over a period of about 100 million years.
At the end of this period, the star is in its horizontal branch phase, named for the position of the point representing the star on the H-R diagram. The star steadily burns helium and hydrogen, and so its temperature, size, and luminosity do not change significantly. This phase lasts for about 10 million years.
Asymptotic giant phase
When all the helium in the core has fused, the core contracts and therefore becomes hotter. The triple-alpha process begins in a shell surrounding the core, and hydrogen fusion continues in a shell surrounding that. Due to the increased energy produced by the burning in the shells, the star's outer layers expand. The star becomes a giant again, but it is bluer and brighter than it was the first time.
On the H-R diagram, the point representing the star has moved upward and to the right along a line known as the asymptotic (as ihm TOT ihk) giant branch (AGB). The star is therefore called an AGB star.
An AGB star's core is so hot and its gravitational grip on its outermost layers is so weak that those layers blow away in a stellar wind. As each layer blows away, a hotter layer is exposed. Thus, the stellar wind becomes even stronger. Out in space, a succession of new, fast winds slam into old, slow winds that are still moving away from the star. The collisions produce dense shells of gas, some of which cool to form dust.
White dwarf phase
A planetary nebula with an unusual textured appearance, the cause of which is unknown. This photo was taken by the Hubble Space Telescope.
A planetary nebula with an unusual textured appearance, the cause of which is unknown. This photo was taken by the Hubble Space Telescope. Image credit: NASA
In just a few thousand years, all but the hot core of an AGB star blows away, and fusion ceases in the core. The core illuminates the surrounding shells. Such shells looked like planets through the crude telescopes of astronomers who studied them in the 1800's. As a result, the astronomers called the shells planetary nebulae -- and today's astronomers still do. The word nebulae is Latin for clouds.
After a planetary nebula fades from view, the remaining core is known as a white dwarf star. This kind of star consists mostly of carbon and oxygen. Its initial temperature is about 100,000 K.
Black dwarf phase
Because a white dwarf star has no fuel remaining for fusion, it becomes cooler and cooler. Over billions of years, it cools more and more slowly. Eventually, it becomes a black dwarf -- an object too faint to detect. A black dwarf represents the end of the life cycle of an intermediate-mass star.
High-mass stars, those with more than 8 solar masses, form quickly and have short lives. A high-mass star forms from a protostar in about 10,000 to 100,000 years.
High-mass stars on the main sequence are hot and blue. They are 1,000 to 1 million times as luminous as the sun, and their radii are about 10 times the solar radius. High-mass stars are much less common than intermediate- and low-mass stars. Because they are so bright, however, high-mass stars are visible from great distances, and so many are known.
A high-mass star has a strong stellar wind. A star of 30 solar masses can lose 24 solar masses by stellar wind before its core runs out of hydrogen and it leaves the main sequence.
As a high-mass star leaves the main sequence, hydrogen begins to fuse in a shell outside its core. As a result, its radius increases to about 100 times that of the sun. However, its luminosity decreases slightly. Because the star is now emitting almost the same amount of energy from a much larger surface, the temperature of the surface decreases. The star therefore becomes redder.
As the star evolves, its core heats up to 100 million K, enough to start the triple-alpha process. After about 1 million years, helium fusion ends in the core but begins in a shell outside the core. And, as in an intermediate-mass star, hydrogen fuses in a shell outside that. The high-mass star becomes a bright red supergiant.
When the contracting core becomes sufficiently hot, carbon fuses, producing neon, sodium, and magnesium. This phase lasts only about 10,000 years. A succession of fusion processes then occur in the core. Each successive process involves a different element and takes less time. Whenever a different element begins to fuse in the core, the element that had been fusing there continues to fuse in a shell outside the core. In addition, all the elements that had been fusing in shells continue to do so. Neon fuses to produce oxygen and magnesium, a process that lasts about 12 years. Oxygen then fuses, producing silicon and sulfur for about 4 years. Finally, silicon fuses to make iron, taking about a week.
Supernovae
At this time, the radius of the iron core is about 1,900 miles (3,000 kilometers). Because further fusion would consume energy, the star is now doomed. It cannot produce any more fusion energy to balance the force of gravity.
When the mass of the iron core reaches 1.4 solar masses, violent events occur. The force of gravity within the core causes the core to collapse. As a result, the core temperature rises to nearly 10 billion K. At this temperature, the iron nuclei break down into lighter nuclei and eventually into individual protons and neutrons. As the collapse continues, protons combine with electrons, producing neutrons and neutrinos. The neutrinos carry away about 99 percent of the energy produced by the crushing of the core.
Now, the core consists of a collapsing ball of neutrons. When the radius of the ball shrinks to about 6 miles (10 kilometers), the ball rebounds like a solid rubber ball that has been squeezed.
All the events from the beginning of the collapse of the core to the rebounding of the neutrons occur in about one second. But more violence is in store. The rebounding of the ball of neutrons sends a spherical shock wave outward through the star. Much of the energy of the wave causes fusion to occur in overlying layers, creating new elements. As the wave reaches the star's surface, it boosts temperatures to 200,000 K. As a result, the star explodes, hurling matter into space at speeds of about 9,000 to 25,000 miles (15,000 to 40,000 kilometers) per second. The brilliant explosion is known as a Type II supernova.
Supernovae enrich the clouds of gas and dust from which new stars eventually form. This enrichment process has been going on since the first supernovae billions of years ago. Supernovae in the first generation of stars enriched the clouds with materials that later went into making newer stars.
Three generations of stars may exist. Astronomers have not found any of what would be the oldest generation, Population III, stars. But they have found members of the other two generations. Population II stars, which would be the second generation, contain relatively small amounts of heavy elements. The more massive ones aged and died quickly, thereby contributing more nuclei of heavy elements to the clouds. For this reason, Population I stars, the third generation, contain the largest amounts of heavy elements. Yet these quantities are tiny compared with the amount of hydrogen and helium in Population I stars. For example, elements other than hydrogen and helium make up from 1 to 2 percent of the mass of the sun, a Population I star.
Neutron stars
After a Type II supernova blast occurs, the stellar core remains behind. If the core has less than about 3 solar masses, it becomes a neutron star. This object consists almost entirely of neutrons. It packs at least 1.4 solar masses into a sphere with a radius of about 6 to 10 miles (10 to 15 kilometers).
Neutron stars have initial temperatures of 10 million K, but they are so small that their visible light is difficult to detect. However, astronomers have detected pulses of radio energy from neutron stars, sometimes at a rate of almost 1,000 pulses per second.
A neutron star actually emits two continuous beams of radio energy. The beams flow away from the star in opposite directions. As the star rotates, the beams sweep around in space like searchlight beams. If one of the beams periodically sweeps over Earth, a radio telescope can detect it as a series of pulses. The telescope detects one pulse for each revolution of the star. A star that is detected in this way is known as a pulsar.
Black holes
If the stellar core remaining after the supernova explosion has about 3 or more solar masses, no known force can support it against its own gravitation. The core collapses to form a black hole, a region of space whose gravitational force is so strong that nothing can escape from it. A black hole is invisible because it traps even light. All its matter is located at a single point in its center. This point, known as a singularity, is much smaller than an atomic nucleus.
Low-mass stars, ranging from 0.1 to 0.5 solar mass, have surface temperatures less than about 4,000 K. Their luminosities are less than 2 percent of the solar luminosity. Low-mass stars use hydrogen fuel so slowly that they may shine as main-sequence stars for 100 billion to 1 trillion years. This life span is longer than the present age of the universe, believed to be 10 billion to 20 billion years. Therefore, no low-mass star has ever died. Nevertheless, astronomers have determined that low-mass stars will never fuse anything but hydrogen. Thus, as these stars die, they will not pass through a red-giant phase. Instead, they will merely cool to become white dwarfs, then black dwarfs.
Binary stars develop from two protostars that form near each other. More than 50 percent of what seem to the unaided eye to be single stars are actually binaries.
Transfer of mass occurs in a binary star system. Matter flows from a sunlike star, in the background in this illustration, to a disk orbiting a white dwarf star, then to the surface of the dwarf.
Transfer of mass occurs in a binary star system. Matter flows from a sunlike star, in the background in this illustration, to a disk orbiting a white dwarf star, then to the surface of the dwarf. Image credit: Space Telescope Science Institute
One star in a binary system can affect the life cycle of the other if the two stars are sufficiently close together. Between the stars is a location called the Lagrange point, named for the French mathematician Joseph Louis Lagrange, where the star's gravitational forces are exactly equal. If one of the stars expands so much that its outer layers pass the Lagrange point, the other star will begin to strip away those layers and accumulate them on its surface.
This process, called mass transfer, can take many forms. Mass transfer from a red giant onto a main-sequence companion can add absorption lines of carbon or other elements to the spectrum of the main- sequence star. But if the stars are close together, the material will flow in the opposite direction when the giant star becomes a white dwarf. The matter will spiral in toward the dwarf, forming a hot disk around it. The disk will flare brilliantly in visible and ultraviolet radiation.
If the giant star leaves behind a neutron star or a black hole instead of a white dwarf, an X-ray binary may form. In this case, the matter transferred from the main-sequence star will become extremely hot. When this matter strikes the surface of the neutron star or is pulled into the black hole, it will emit X rays.
In a third case, the red giant becomes a white dwarf, and the main-sequence star becomes a red giant. When enough gas from the giant accumulates on the dwarf's surface, gas nuclei will fuse violently in a flash called a nova. In some cases, so much gas will accumulate that its weight will cause the dwarf to collapse. Almost instantly, the dwarf's carbon will fuse, and the entire dwarf will explode in a Type I supernova. This kind of explosion is so bright that it can outshine an entire galaxy for a few months.
Contributor: Paul J. Green, Ph.D., Astrophysicist, Smithsonian Astrophysical Observatory.
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