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hot incandescent sphere of gas, held together by its own gravitation, and emitting light and other forms of electromagnetic radiation whose ultimate source is nuclear energy.
..... Click the link for more information. that is relatively cool but very luminous because of its great size. All normal stars are expected to pass eventually through a red-giant phase as a consequence of stellar evolutionstellar evolution,
life history of a star, beginning with its condensation out of the interstellar gas (see interstellar matter) and ending, sometimes catastrophically, when the star has exhausted its nuclear fuel or can no longer adjust itself to a stable configuration.
..... Click the link for more information. . As a star uses up its hydrogen by converting it to helium, its central core contracts while the outer layers expand and cool; this process produces the low temperature and large size (from 10 to 1,500 times that of the sun) that characterize the red giant. Although most giant stars are red, some prominent giant stars are other colors near the red end of the spectrum, e.g., ArcturusArcturus
, brightest star in the constellation Boötes and 4th-brightest star in the entire sky; Bayer designation Alpha Boötis; 1992 position R.A. 14h15.3m, Dec. +19°13'.
..... Click the link for more information. (orange), AldebaranAldebaran
, brightest star in the constellation Taurus; Bayer designation α Tauri; 1992 position R.A. 4h35.5m, Dec. +16°30'. An orange giant star (spectral class K5 III) with apparent magnitude averaging 0.
..... Click the link for more information. (orange), and CapellaCapella,
brightest star in the constellation Auriga; Bayer designation α Aurigae; 1992 position R.A. 5h16.1m, Dec. +45°59'. Capella is a yellow giant star of spectral class G8 III and is also a spectroscopic binary star with a component of spectral
..... Click the link for more information. (yellow). The largest and brightest stars (excluding supernovassupernova,
a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than
..... Click the link for more information. ) are classed as supergiants. Blue supergiants, e.g., RigelRigel
, bright star in the constellation Orion; Bayer designation Beta Orionis; 1992 position R.A. 5h14.2m, Dec. −8°13'. A huge, blue supergiant of spectral class B8 Ia, Rigel has an intrinsic brightness about 40,000 times as luminous as that of
..... Click the link for more information. , are young stars on the main sequence of the Hertzsprung-Russell diagramHertzsprung-Russell diagram
[for Ejnar Hertzsprung and H. N. Russell], graph showing the luminosity of a star as a function of its surface temperature. The luminosity, or absolute magnitude, increases upwards on the vertical axis; the temperature (or some temperature-dependent
..... Click the link for more information. , whereas red supergiants, e.g., BetelgeuseBetelgeuse
, bright star in the constellation Orion; Bayer designation α Orionis; 1992 position R.A. 5h54.8m, Dec. +7°24'. A red supergiant with a luminosity about 13,000 times that of the sun, it is of spectral class M2 Iab.
..... Click the link for more information. and AntaresAntares
, brightest star in the constellation Scorpius; Bayer designation Alpha Scorpii; 1992 position R.A. 16h27.6m, Dec. −26°22'. A red supergiant of spectral class M1, Antares has an apparent magnitude of about 0.
..... Click the link for more information. , are old, highly evolved stars.
red giantA giant star with a surface temperature between 2000–4000 kelvin and a diameter 10–1000 times greater than the Sun. Red giants are one of the final phases in the evolution of a normal star, reached when its central hydrogen has been used up. The star resorts to burning hydrogen in a shell around its dense inert helium core, which results in a rapid inflation of the outer layers of atmosphere. During subsequent evolution, interior structural and nuclear changes cause the star's temperature, size, and luminosity to alter (see giant); the star may change to a more compact hotter type of giant, then swell back to a red giant, more than once as it evolves. Because red giants are so distended, gravity has only a small effect on their surface layers and many lose considerable amounts of mass into space in the form of stellar winds. Condensing dust grains around the stars make them infrared sources, while molecules here can emit maser radiation, making them maser sources.
Red giants are often variable stars since their surface layers slowly expand and contract. Because these stars are so large, their pulsations usually take about a year to complete; red giants therefore belong to the class of Mira stars, or long-period pulsating variables. Low-mass red giants end up as planetary nebulae, gently puffing off their distended atmospheres at low velocities (a few km s–1) and leaving their dense cores exposed as white dwarf stars, radiating away their heat into space. More massive red giants explode as Type II supernovae. See also stellar evolution; supergiant.