Star (astronomy)

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Red Giants and Supergiants

The next stage of a star’s evolution involves dramatic stages of expansion and contraction as the star approaches the end of its life cycle. After the star has consumed the hydrogen in its innermost core, the core begins shrinking, converting hydrogen into helium in ever-larger shells around the inner core. The star’s core shrinks because the outward pressure of heat generated by the nuclear reactions no longer balances the inward gravitational attraction of the star’s mass for itself.

Although the core of a star gradually shrinks as it exhausts its hydrogen supply, the star itself begins expanding. It resorts to burning the hydrogen in a shell around its helium core, which inflates the outer layers of its atmosphere. Eventually, the star expands into a red giant, possibly attaining a diameter from 10 to 1,000 times the diameter of the Sun. For example, in its red giant stage, the Sun will expand to the size of the orbit of Earth or beyond and become 2,000 times brighter than it is now. The shrinking core increases the star's internal pressure. The increase in pressure makes the star's temperature increase again until it is hot enough to trigger nuclear reactions between previously inert helium nuclei present in the star. This new series of nuclear reactions releases more energy and the star's core stops contracting. At this point, the star's outer atmosphere begins to contract.

Although rare, the most massive stars can evolve into stars called supergiants. In such a star, radiation released by the fusion of helium into carbon causes the red giant to expand into a supergiant—a star at least 500 times the Sun’s size.

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White Dwarf

When a low- to medium-mass star exhausts the nuclear fuel in its core, it collapses under the gravitational pressure of its own weight into an extremely compact, dense star known as a white dwarf. As a more massive star (6 to 8 solar masses) collapses to a white dwarf, it blows off more than half of its outer layer into space as a planetary nebula—gas and dust that may provide building material for planets in newly forming solar systems. Although dimmer than the original star, a white dwarf will continue radiating light for several billion years from thermal energy (heat energy) trapped in its interior.

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Most white dwarfs have only slightly larger radii than that of Earth, but the density of a typical white dwarf is about 600,000,000 kg/m³ (4,000,000 lbs/ft³), and the mass of a typical white dwarf is about 70 percent of the Sun’s mass. As a white dwarf slowly loses energy and cools, it changes color from blue-white, to white, to yellow, to orange, and finally to dull red. After several billion years, the white dwarf exhausts its energy supply, and becomes what is known as a black dwarf.

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Neutron Stars and Black Holes

When the core of a supergiant has exhausted its helium, the core will again contract, and if the core is sufficiently massive, additional nuclear reactions will be triggered during this contraction. These nuclear reactions convert carbon and other elements into increasingly heavier elements, until the core may consist largely of iron. Some supergiant stars then form an astronomical body known as a neutron star. Neutron stars form when a supergiant continues to collapse and the material in the stellar core becomes more and more dense. The atomic nuclei are forced so close together that they fuse to form neutrons. When this occurs, the core stops contracting and remains as a neutron star, a rapidly spinning, extremely dense star consisting mainly of closely packed neutrons. A neutron star may contain a mass that is equal to 1.4 to 3 times the Sun’s mass and that is compressed into a volume about 20 km (about 10 mi) in diameter.

Still more massive supergiants, with a mass more than 5 times that of the Sun, may continue collapsing until their nuclei are crushed into even denser matter. This matter forms a body so dense that it forms a black hole—an extremely dense, invisible celestial body with a gravitational field powerful enough to prevent the escape of light.

During the collapse of a supergiant, the outer layers of the star are ejected into space by a massive explosion known as a supernova (for more information, see the Supernova section of this article). This ejected gas and dust contain hydrogen and heavier elements, such as carbon, oxygen, nitrogen, and iron, that formed in the supergiant’s core. Supergiants are a major source of heavy elements throughout the universe. Astronomers believe Earth and all its living organisms are composed of elements formed in the interiors of stars, especially supergiants that exploded as supernovas.

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Age

Astronomers have identified stars that are as young as 25,000 years old and others that are more than 10 billion years old. The Sun is 4.6 billion years old. Astronomers believe that once medium-sized stars are fully formed, they may last up to 10 billion years. While the Sun’s core will probably run out of hydrogen in about 7 billion years, the very hottest stars spend their energy much faster and die—or become dark and cool—much more rapidly.

The locations of different stars can help astronomers determine the age of these stars. Most O (blue) and B (blue-white) main-sequence stars are not randomly distributed throughout the sky. Instead, these stars tend to be grouped into associations lying along the spiral arms of the Milky Way. Some of these groups, such as one in the constellation Perseus, appear to be expanding. By extrapolating its expanding motion backward, astronomers can determine that the age of the expansion (and therefore the stars) is less than two million years. On an astronomical timescale, O and B stars are extremely young.

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Important Types of Peculiar Stars

Although most stars are normal members of the main sequence, astronomers have also identified stars with variations in brightness (known as variable stars) and stars with unusual spectra.

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