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18 Mar 15
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A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses (M☉)) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, from 5,000 K and lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars.
The most common red giants are stars nearing the end of the so-called red-giant-branch (RGB) but are still fusing hydrogen into helium in a shell surrounding a degenerate helium core. Other red giants are: the red clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process; and the asymptotic-giant-branch (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and sometimes with a hydrogen burning shell just beyond that.[1]
The nearest red giant is Gamma Crucis, 88 light years away, but the orange giant Arcturus is described by some as a red giant and it is 36 light years away.
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07 Mar 14
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diameters about 20–100 times the Sun
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11 Dec 13
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10 Dec 13
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When a star initially forms from a collapsing molecular cloud in the interstellar medium, it contains primarily hydrogen and helium
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Red giants are evolved from main-sequence stars with masses in the range from about 0.3M☉ to around 8M☉
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with trace amounts of "metals" (in stellar structure, this simply refers to any element that is not hydrogen or helium i.e. atomic number greater than 2). These elements are all uniformly mixed throughout the star. The star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen (a few million kelvin) and establishes hydrostatic equilibrium.
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Over its main sequence life, the star slowly converts the hydrogen in the core into helium; its main-sequence life ends when nearly all the hydrogen in the core has been fused. For the Sun, the main-sequence lifetime is approximately 10 billion years. More-massive stars burn disproportionately faster and so have a shorter lifetime than less massive stars
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When the star exhausts the hydrogen fuel in its core, nuclear reactions can no longer continue and so the core begins to contract due to its own gravity. This brings additional hydrogen into a zone where the temperature and pressure are adequate to cause fusion to resume in a shell around the core. The higher temperatures lead to increasing reaction rates, enough to increase the star's luminosity by a factor of 1,000–10,000. The outer layers of the star then expand greatly, thus beginning the red-giant phase of the star's life. As the star expands, the energy produced in the burning shell of the star is spread over a much larger surface area, resulting in a lower surface temperature and a shift in the star's visible light output towards the red – hence it becomes a red giant
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In stars massive enough to ignite helium fusion, an analogous process occurs when the central helium is exhausted and the star collapses once again, causing helium in an outer shell to begin fusing. At the same time hydrogen may begin fusion in a shell just outside the burning helium shell. This puts the star onto the asymptotic giant branch, a second red-giant phase.[9] The helium fusion results in the build up of a carbon–oxygen core. A star below about 8 M☉[7] will never start fusion in its degenerate carbon–oxygen core. Instead, at the end of the asymptotic-giant-branch phase the star will eject its outer layers, forming a planetary nebula with the core of the star exposed, ultimately becoming a white dwarf
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If the star has about 0.2 to 0.5 M☉,[7] it is massive enough to become a red giant but does not have enough mass to initiate the fusion of helium.[6] These "intermediate" stars cool somewhat and increase their luminosity but never achieve the tip of the red-giant branch and helium core flash. When the ascent of the red-giant branch ends they puff off their outer layers much like a post-asymptotic-giant-branch star and then become a white dwarf.
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05 Nov 10
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hydrogen fuel in its core, nuclear reactions in the core stop, so the core begins to contract due to its gravity
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27 Sep 10
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A red giant is a luminous giant star of low or intermediate mass (roughly 0.5–10 solar masses) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower. The appearance of the red giant is from yellow orange to red, including the spectral types K and M, but also class S stars and most carbon stars.
The most common red giants are the so-called red giant branch stars (RGB stars) whose shells are still fusing hydrogen into helium, while the core is inactive helium. Another case of red giants are the asymptotic giant branch stars (AGB) that produces carbon from helium by the triple-alpha process.[1] To the AGB stars belong the carbon stars of type C-N and late C-R.
Prominent bright red giants in the night sky include Aldebaran (Alpha Tauri), Arcturus (Alpha Bootis), and Gamma Crucis (Gacrux), while the even larger Antares (Alpha Scorpii) and Betelgeuse (Alpha Orionis) are red supergiants.
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Red giants are stars with radii tens to hundreds of times larger than that of the Sun which have exhausted the supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core. The main sequence stars of spectral types A through K are believed to become red giants.[2]
In fact, such stars are not big red spheres with sharp limbs (when one is close to it) as displayed on many images. Due to the very low density such stars may not have a sharp photosphere but a star body which gradually transfers into a 'corona'.[3][4]
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] However the results of studies announced in 2008 show that due to tidal interaction between sun and Earth, Earth would actually fall back into a lower orbit, and get engulfed and incorporated inside the sun before the sun reaches its largest size, despite the sun losing about 38% of its mass.[17] Before this happens, Earth's biosphere will have long been destroyed by the Sun's steady increase in brightness as its hydrogen supply dwindles and its core contracts, even before the transition to a Red Giant. After just over 1 billion years, the extra solar energy input will cause Earth's oceans to evaporate and the hydrogen from the water to be lost permanently to space, with total loss of water by 3 billion years.[18] Earth's atmosphere and lithosphere will become like that of Venus. Over another billion years, most of the atmosphere will get lost in space as well;[15] ultimately leaving Earth as a desiccated, dead planet with a surface of molten rock.
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13 Mar 07
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10 Nov 06
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