Red giant
From Free net encyclopedia
According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. Examples include Aldebaran and Arcturus.
They are stars of 0.4 - 8 times the mass of the Sun which have exhausted their supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core. Since the inert helium core has no source of energy of its own, it contracts and heats up, and its gravity compresses the hydrogen in the layer immediately above it, thus causing it to fuse faster. This in turn causes the star to become more luminous (from 1,000 – 10,000 times brighter) and expand; the degree of expansion outstrips the increase in luminosity, thus causing the effective temperature to decrease. In stars massive enough to ignite helium fusion, an analogous process occurs when central helium is exhausted and the star switches to fusing helium in a shell, although with the additional complication that in many cases hydrogen fusion will continue in a shell at lesser depth &emdash; this puts stars onto the asymptotic giant branch.[1],[2],[3] The decrease in surface temperature shifts the star's visible light output to the red — hence red giant. Stars of spectral types O through K are believed to become red giants (or supergiants in the case of O and B stars).
Very low mass stars are thought to be fully convective[4], and thus may not accumulate an inert core of helium, and thus may exhaust all of their fuel without ever becoming red giants.[5]
If the star is less than 2.57 solar masses, the addition of helium to the core by shell hydrogen fusing will cause a helium flash—a rapid burst of helium fusing in the core, after which the star will commence a brief period of helium fusing before beginning another ascent of the red giant branch. Stars more massive than 2.5 solar masses enter the helium fusing phase of their lives much more smoothly. The core helium fusing phase of a star's life is called the horizontal branch in metal-poor stars, so named because these stars lie on a nearly horizontal line in the Hertzsprung-Russell diagram of many star clusters. Metal-rich helium-fusing stars do not lie on a horizontal branch, but instead lie in a clump (the red clump) in the Hertzsprung-Russell diagram.
Earth's Sun
As Earth's sun is of one solar mass, it is expected to become a red giant in about five billion years. It will become sufficiently large to engulf the current orbits of the solar system's inner planets, including Earth. However, the gravitational pull of the sun will have weakened by then due to its loss of mass, and it is possible (but unlikely) that Earth may escape to a wider orbit.
Red giants in fiction
- In the Superman comic books, the doomed planet Krypton orbited a red giant. Current continuity attributes the source of Superman's powers to Earth's yellow sun.
See also
cs:Červený obr da:Rød kæmpe de:Roter Riese es:Gigante roja fr:Géante rouge ko:적색거성 it:Gigante rossa he:ענק אדום lt:Raudonoji milžinė hu:Vörös óriás nl:Rode reus ja:赤色巨星 pl:Czerwony olbrzym pt:Estrela vermelha gigante ru:Красный гигант sk:Červený obor sl:Rdeča orjakinja fi:Punainen jättiläinen sv:Röd jätte zh:紅巨星