A small number of progenitors of type II-L and type IIb supernovae have been observed, all having luminosities around 100,000 L☉ and somewhat higher temperatures up to 6,000K. Red supergiants develop from main-sequence stars with masses between about 8 M☉ and 30 M☉. Lower-mass stars develop a degenerate helium core during a red giant phase, undergo a helium flash before fusing helium on the horizontal branch, evolve along the AGB while burning helium in a shell around a degenerate carbon-oxygen core, then rapidly lose their outer layers to become a white dwarf with a planetary nebula.  As a red giant, the Sun will grow so large that it will engulf Mercury, Venus, and probably Earth. The K-type stars, especially early or hotter K types, are sometimes described as orange supergiants (e.g.  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 H–R diagram of many star clusters. The subsequent cascade of events leads, eventually to a Type II supernova event. Red supergiants are cool and large. The best known red supergiant is Betelgeuse, with a luminosity about 10,000 times that of the Sun. In the latest stages of mass loss, before a star explodes, surface helium may become enriched to levels comparable with hydrogen. A huge amount of energy pushes the outer layers of the star outwards and it turns into a red supergiant. They have spectral types of K and M, hence surface temperatures below 4,100 K. They are typically several hundred to over a thousand times the radius of the Sun, although size is not the primary factor in a star being designated as a supergiant. The hydrogen-burning shell results in a situation that has been described as the mirror principle; when the core within the shell contracts, the layers of the star outside the shell must expand. It changes more drastically than its sun-like siblings and becomes a red supergiant. However, for planets orbiting a 0.5 M☉ star in equivalent orbits to those of Jupiter and Saturn they would be in the habitable zone for 5.8 billion years and 2.1 billion years, respectively; for stars more massive than the Sun, the times are considerably shorter. The biggest stars in the universe are called red supergiants.To get a picture of how huge these stars are, just imagine being able to fit 1800 suns into one red supergiant or imagine the sun growing so much that it already reaches the orbit of Saturn. , The evolutionary path the star takes as it moves along the red-giant branch depends on the mass of the star. Because of its higher mass, when the core collapses after the hydrogen burning phase the rapidly increased temperature leads to the fusion of helium very quickly. The stellar limb of a red giant is not sharply defined, contrary to their depiction in many illustrations.  The helium fusion results in the build up of a carbon–oxygen core. Eventually, it will exhaust all its nuclear fuel that runs the star. 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. Very-low-mass stars are fully convective and may continue to fuse hydrogen into helium for up to a trillion years until only a small fraction of the entire star is hydrogen. , 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. Their luminosity increases by a factor of about three.  Observations have also provided evidence of a hot chromosphere above the photosphere of red giants, where investigating the heating mechanisms for the chromospheres to form requires 3D simulations of red giants. Eventually the level of helium increases to the point where the star ceases to be fully convective and the remaining hydrogen locked in the core is consumed in only a few billion more years. A red giant is a star that has exhausted the supply of hydrogen in its core and has begun thermonuclear fusion of hydrogen in a shell surrounding the core. For example, Alpha Herculis is classified as a giant star with a radius of between 264 to 303 R☉ while Epsilon Pegasi is a K2 supergiant of only 185 R☉.  The coolest red giants have complex spectra, with molecular lines, emission features, and sometimes masers, particularly from thermally pulsing AGB stars. Red giants with known planets: the M-type HD 208527, HD 220074 and, as of February 2014, a few tens of known K-giants including Pollux, Gamma Cephei and Iota Draconis. The changes they experience are called "stellar evolution". , The observed progenitors of type II-P supernovae all have temperatures between 3,500K and 4,400K and luminosities between 10,000 L☉ and 300,000 L☉. When pre-red supergiant stars leave the main sequence, oxygen is more abundant than carbon at the surface, and nitrogen is less abundant than either, reflecting abundances from the formation of the star.  There is an upper limit to the luminosity and radius of a red supergiant at around 320,000 or 630,000 L☉ and around 1,500 R☉. At that point, a star is said to have moved off the main sequence. In theoretical extreme mass loss models, sufficient hydrogen may be lost that helium becomes the most abundant element at the surface. While many red supergiants will not experience a blue loop, some can have several. Left behind will be the core of the star, having been compressed due to the immense gravitational pressure into a neutron star; or in the cases of the most massive of stars, a black hole is created. Eventually, what's left of the star shrinks to become a slowly cooling white dwarf. Stars go through specific steps throughout their lives. As a result, while red supergiants are counted as the largest stars in the universe, they are not the most massive because they lose mass as they age, even as they expand outward. The intermediate class Iab is also used. , As of June 2014, fifty giant planets have been discovered around giant stars. Metal-rich helium-fusing stars instead lie on the so-called red clump in the H–R diagram.. , Many of the well-known bright stars are red giants, because they are luminous and moderately common.
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