Stars

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

Bibliography

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. <http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html>.