RSGC1 contains at least 12 red supergiants, RSGC2 (also known as Stephenson 2) contains at least 26 (Stephenson 2-18, one of the stars, is possibly the largest star known), RSGC3 contains at least 8, and RSGC4 (also known as Alicante 8) also contains at least 8. "Chromospheric Activity in Red Giants, and Related Phenomena", "The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA", "ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse", "Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars - VI. The Yerkes or Morgan-Keenan (MK) classification system[3] is almost universal. Main-sequence stars more massive than about 40 M☉ do not expand and cool to become red supergiants. They will reach late K or M class and become a red supergiant. [18] Currently this has been applied mainly to individual objects, but it may become useful for analysis of galactic structure and discovery of otherwise obscured red supergiant stars. Although traditionally it has been suggested the evolution of a star into a red giant will render its planetary system, if present, uninhabitable, some research suggests that, during the evolution of a 1 M☉ star along the red-giant branch, it could harbor a habitable zone for several billion years at 2 astronomical units (AU) out to around 100 million years at 9 AU out, giving perhaps enough time for life to develop on a suitable world. Luminosity and temperature steadily increase during this time, just as for more-massive main-sequence stars, but the length of time involved means that the temperature eventually increases by about 50% and the luminosity by around 10 times. 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 exact reasons for blue loops vary in different stars, but they are always related to the helium core increasing as a proportion of the mass of the star and forcing higher mass-loss rates from the outer layers. Betelgeuse and Antares are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars. To understand what they are, it's important to know how stars change over time. Metal-rich helium-fusing stars instead lie on the so-called red clump in the H–R diagram.[13]. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K (4,700 °C; 8,500 °F) or lower. The K-type stars, especially early or hotter K types, are sometimes described as orange supergiants (e.g. These become cool helium white dwarfs. [10], 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. K-type supergiants are uncommon compared to M-type because they are a short-lived transition stage and somewhat unstable. However, these stars have a very loose definition, they are usually just red (or sometimes blue) supergiant stars that are the highest order: the most massive and the largest. When astronomers look at the largest stars (by volume) in the universe, they see a great many red supergiants. That means the nuclear fusion in their cores (where they fuse hydrogen to create helium) provides enough energy and pressure to keep the weight of their outer layers from collapsing inwards. [14], The spectra of red supergiants are similar to other cool stars, dominated by a forest of absorption lines of metals and molecular bands. Stars go through specific steps throughout their lives. Zeta Cephei), or even as yellow (e.g. However, these giant planets are more massive than the giant planets found around solar-type stars. After some billions more years, they start to become less luminous and cooler even though hydrogen shell burning continues. Intermediate "super-AGB" stars, around 9 M☉, can undergo carbon fusion and may produce an electron capture supernova through the collapse of an oxygen-neon core. Temperatures can reach 10,000K at the peak of the blue loop. Red giants differ in a way by which they generate energy: Many of the well-known bright stars are red giants, because they are luminous and moderately common. The red-giant branch variable star Gamma Crucis is the nearest M-class giant star at 88 light-years. A star below about 8 M☉ will never start fusion in its degenerate carbon–oxygen core. At that point, a star is said to have moved off the main sequence. [9] When a star initially forms from a collapsing molecular cloud in the interstellar medium, it contains primarily hydrogen and helium, with trace amounts of "metals" (in stellar structure, this simply refers to any element that is not hydrogen or helium i.e. Researchers now prefer to categorize these as AGB stars distinct from supergiants because they are less massive, have different chemical compositions at the surface, undergo different types of pulsation and variability, and will evolve in a different way, usually producing a planetary nebula and white dwarf.