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Stars are huge balls of very hot, mostly ionized, gas (plasma) that are held together by gravity. They form when vast agglomerations of gas and dust known as molecular clouds (typically 10 to 100 light years across) fragment into denser cores (tenths of a light-year across) that can collapse inward under their own gravity. Matter falling inward forms one or more dense, hot, central objects known as protostars. Rotation forces some of the matter to accumulate in a disk rotating around the protostar(s). As gravity pulls rotating material inward, it spins faster, akin to what happens to figure skaters when they pull their initially outstretched arms in toward their bodies.

In order for material to fall onto a protostar from a rapidly spinning disk, it must slow down. Recent theoretical work suggests that this is accomplished through the interaction of the material with magnetic fields that thread the disks of protostars. Near the disk, the magnetic field is bent into an hourglass shape. Gas particles are flung off the rotating disk by centrifugal force, slowing the rotation of the disk. The ejected material is channeled into narrow jets perpendicular to the disk, while material from the disk falls onto the protostar. Planets may eventually form within the disk. The jets plow into the surrounding medium, sweeping up a bipolar outflow on opposite sides of the protostar. It is not yet known whether the final mass of a star is determined by the initial mass of the core in which it was born or from the clearing of material by bipolar outflows. In any case, the final mass of the star determines how it will evolve from this point on.

Main Sequence Stars

When the star has accumulated enough material so that the temperature and pressure are high enough, nuclear fusion reactions, which convert hydrogen into helium, begin deep within the core of the star. The energy from the reactions makes its way to the surface of the star in about a million years, causing the star to shine. The pressure from these nuclear reactions at the star's core balances the pull of gravity, and the star is now called a main sequence star.

This name is derived from the relationship between a star's intrinsic brightness and its temperature, which was discovered independently by Danish astronomer Ejnar Hertzsprung (in 1911) and American astronomer Henry Norris Russell (in 1913). This relationship is displayed in a Hertzsprung-Russell diagram. A star's color depends on its surface temperature; red stars are the coolest and blue stars are the hottest. The temperature, brightness, and longevity of a star on the main sequence are determined by its mass; the least massive main sequence stars are the coolest and dimmest, and the most massive stars are the hottest and brightest. Objects less than about one-thirteenth the mass of the Sun can never sustain fusion reactions. These objects are known as brown dwarfs.

Red Giants and Red Supergiants

Counterintuitively, the more massive a star is, the more rapidly it uses up the hydrogen at its core. The most massive stars deplete their central hydrogen supply in a million years, whereas stars that are only about one-tenth the mass of the Sun remain on the main sequence for hundreds of billions of years. When hydrogen becomes depleted in the core, the core starts to collapse, and the temperature and pressure rise, so that fusion reactions can begin in a shell around the helium core. This new heat supply causes the outer layers of the star to expand and cool, and the star becomes a red giant, or a red supergiant if it is very massive.

Planetary Nebulae, White Dwarfs, and Black Dwarfs

Once stars up to a few times the mass of the Sun reach the red giant phase, the core continues to contract and temperatures and pressures in the core become high enough for helium nuclei to fuse together to form carbon. This process occurs rapidly (only a few minutes in a star like the Sun), and the star begins to shed the outer layers of its atmosphere as a diffuse cloud called a planetary nebula. Eventually, only about 20 percent of the star's initial mass remains in a very dense core, about the size of Earth, called a white dwarf. White dwarfs are stable because the pressure of electrons repulsing each other balances the pull of gravity. There is no fuel left to burn, so the star slowly cools over billions of years, eventually becoming a cold, dark object known as a black dwarf.

Supernovae, Neutron Stars, and Black Holes

After a star more than about five times the mass of the Sun has become a red supergiant, its core goes through several contractions, becoming hotter and denser each time, initiating a new series of nuclear reactions that release energy and temporarily halt the collapse. Once the core has become primarily iron, however, energy can no longer be released through fusion reactions, because energy is required to fuse iron into heavier elements. The core then collapses violently in less than a tenth of a second.

The energy released from this collapse sends a shock wave through the star's outer layers, compressing the material and fusing new elements and radioactive isotopes, which are propelled into space in a spectacular explosion known as a supernova. This material seeds space with heavy elements and may collide with other clouds of gas and dust, compressing them and initiating the formation of new stars. The core that remains behind after the explosion may become either a neutron star, as the intense pressure forces electrons to combine with protons , or a black hole, if the original star was massive enough so that not even the pressure of the neutrons can overcome gravity. Black holes are stars that have literally collapsed out of existence, leaving behind only an intense gravitational pull.

see also Astronomer (volume 2); Astronomy, Kinds of (volume 2); Black Holes (volume 2); Galaxies (volume 2); Gravity (volume 2); Pulsars (volume 2); Sun (volume 2); Supernova (volume 2).

Grace Wolf-Chase


Bennett, Jeffrey, Megan Donahue, Nicholas Schneider, and Mark Voit. The Cosmic Perspective. Menlo Park, CA: Addison Wesley Longman, 1999.

Kaler, James B. Stars. New York: Scientific American Library and W. H. Freeman,1992.

Seeds, Michael A. Horizons: Exploring the Universe, 6th ed. Pacific Grove, CA:Brooks/Cole, 2000.

Internet Resources

Imagine the Universe! NASA Goddard Space Flight Center. <>.

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Remote yet familiar, stars have fascinated people throughout history and are part of many myths and legends. Although the sun and the moon usually have the leading roles in mythology, often appearing as deities, the stars also appear in many stories. In some cultures, the stars represent part of the cosmos, such as the heavens or the home of the gods, or a path between the earth and another world. In many myths and legends, individual stars or constellations, groups of stars, have special significance.

deity god or goddess

cosmos the universe, especially as an orderly and harmonious system

Explaining the Stars. People who lived before electric lights and air pollution dimmed the night skies saw the heavens glittering

* See Names and Places at the end of this volume for further information.

with thousands of stars. They developed various stories to explain their brilliant presence.

The Paiute of North America describe the stars as the children of the sun and moon. Because the sun loves to eat his children, the stars disappear whenever he rises above the horizon. However, the moon, their mother, often dances happily across the sky with the stars. To the Yakut of Siberia, the stars are crystal windows that allow the gods to look down at earth. The tent-dwelling Turko-Tatar people of Central Asia picture the sky as a large tent over the earth, with the stars as tiny holes in the tent.

The Milky Way, a dense band of stars that spans the sky, marks the center of the galaxy to which our solar system belongs. In myths, however, the Milky Way has been a road, a river, and a bridge between worlds. According to a Peruvian tradition, the Vilcanota River is a reflection of the Milky Way and water constantly circulates between the two, passing from the river to heaven and back again. The Navajo say that the trickster Coyote created the Milky Way by tossing a blanket full of sparkling stone chips into the sky. Scattered in a great arc, the stones formed a pathway linking heaven and earth.

In many traditions, the stars have been associated with death and the afterlife. The Maya considered the Milky Way to be the road to Xibalba, the underworld. Many Native Americans regard the Milky Way as the path followed by the souls of the dead. According to the Zulu and Ndebele people of southern Africa, the stars are the eyes of dead ancestors, keeping watch on the living from above.

Constellations and Individual Stars in Myths. Chinese mythology includes many references to the stars. Various deities, such as the god of literature and the god of long life, were associated with the stars. One myth that occurs in several versions concerns the Weaver Girl, the goddess who weaves the clouds, and the Herdsman, who tends the cattle of heaven. The two were lovers. When the gods placed them in the sky, the Weaver Girl became the star called Vega, while the Herdsman became either the star Altair or the constellation Aquila. The gods separated the lovers with the river of the Milky Way so that they would not neglect their work. But every year, on the seventh night of the seventh month, birds formed a bridge across the Milky Way allowing the Weaver Girl and the Herdsman to meet.

Berenice's Hair

Berenice was the wife of Ptolemy III, a king of Egypt, in the 200s b.c. According to legend, she promised to sacrifice her hair to the goddess Venus if the gods brought her husband safely back from war. When Ptolemy returned unharmed, Berenice cut off her hair and placed it in Venus's temple as promised, but the hair disappeared. The royal astronomer told Ptolemy that it had become a constellation in the night sky, known to this day as the Coma Berenices, or Berenice's Hair.

trickster mischievous figure appearing in various forms in the folktales and mythology of many different peoples

underworld land of the dead

nymph minor goddess of nature, usually represented as young and beautiful

People have told many tales about the group of seven stars called the Pleiades. In Greek mythology these stars were the seven daughters of the Titan Atlas* and the ocean nymph Pleione. According to some accounts, Zeus* placed them in the sky to protect them from the hunter Orion. But then Orion became a constellation and continued to chase the Pleiades across the heavens. The cattle-herding Masai people of Africa see the Pleiades as a group of cattle, and their appearance in the sky marks the rainy season. The Inca of South America called the Pleiades Collca meaning a place where grain is storedand believed that the constellation protected seeds and farming.

The constellation Ursa Major, called the Great Bear or the Big Dipper, appears in many Native American myths. The Seneca of New York believed that the constellation was made up of a large bear and the six hunters who chased it into the sky. The Inuit of northern Greenland, though, see Ursa Major as a giant caribou. They imagine the constellation Orion as a series of steps in the great bank of snow that connects earth and heaven.

The constellation known as the Southern Cross figures in Australian mythology. According to a story from New South Wales, it is a gum tree in which a man and a spirit are trapped, their eyes blazing forth as stars. Two other stars are white birds that rose into the sky with the tree.

Many myths and legends refer to the morning star and the evening star. These are not true stars but names for the planet Venus, which shines brightly near the horizon early or late in the night, depending on the time of year. A Norse* myth says that the morning star was originally the toe of a hero named Aurvandil. Thor* had carried Aurvandil out of Giantland and across the river Elivagar. On the way, however, one of the hero's toes froze, so Thor broke it off and threw it into the sky. To the Greeks, the evening star was Hesperus, grandfather of the goddesses called the Hesperides, who guarded the golden apples of eternal life on islands in the western sea.

See also Moon; Sun.

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