Larger stars are more luminous at a given temperature and can now be grouped into bands of differing luminosity.[2]. The Yerkes or Morgan-Keenan (MK) classification system[3] is almost universal. Carbon and oxygen are quickly depleted and nitrogen enhanced as a result of the dredge-up of CNO-processed material from the fusion layers. The luminosity differences between stars are most apparent at low temperatures, where giant stars are much brighter than main-sequence stars. They will reach late K or M class and become a red supergiant. They are already very luminous stars of 10,000-100,000 L☉ due to rapid CNO cycle fusion of hydrogen and they have fully convective cores. The K-type stars, especially early or hotter K types, are sometimes described as orange supergiants (e.g. K-type supergiants are uncommon compared to M-type because they are a short-lived transition stage and somewhat unstable. red giant and supergiant stars (luminosity classes I through III) occupy the region above the main sequence. Red supergiants are supergiant stars of spectral type K-M and a luminosity class of I. The Hertzsprung-Russell diagram the various stages of stellar evolution. [19], Surface abundances of red supergiants are dominated by hydrogen even though hydrogen at the core has been completely consumed. The Hertzsprung-Russell diagram (HR diagram) is one of the most important tools in the study of stellar evolution. [6] These hypergiant spectral classifications are very rarely applied to red supergiants, although the term red hypergiant is sometimes used for the most extended and unstable red supergiants like VY Canis Majoris and NML Cygni. Betelgeuse and Antares are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars Red supergiants are rare stars, but they are visible at great distance and are often variable so there are a number of well-known naked-eye examples: Other examples have become known on account of their enormous size, more than 1,000 R☉: A survey expected to capture virtually all Magellanic Cloud red supergiants[32] detected around a dozen M class stars Mv−7 and brighter, around a quarter of a million times more luminous than the Sun, and from about 1,000 times the radius of the Sun upwards. They are the largest stars in the universe in terms of physical size, although they are not the most massive. [14], The spectra of red supergiants are similar to other cool stars, dominated by a forest of absorption lines of metals and molecular bands. Models indicate that even rapidly rotating main-sequence stars should be braked by their mass loss so that red supergiants hardly rotate at all. An example star is HD 93129 B. Stars with even less hydrogen remaining may produce the uncommon type IIb supernova, where there is so little hydrogen remaining that the hydrogen lines in the initial type II spectrum fade to the appearance of a Type Ib supernova. 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. It groups stars into five main luminosity groups designated by roman numerals: Specific to supergiants, the luminosity class is further divided into normal supergiants of class Ib and brightest supergiants of class Ia. 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. Questions? Helium fusion in the core begins smoothly either while the star is expanding or once it is already a red supergiant, but this produces little immediate change at the surface. 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. The four largest known red supergiants in the Galaxy are Mu Cephei, KW Sagitarii, V354 Cephei, and KY Cygni, which all have radii about 1500 times that of the sun (about 7 astronomical units, or 7 times as far as the Earth is from the sun). Less massive stars may develop a supergiant spectral luminosity class at relatively low luminosity, around 1,000 L☉, when they are on the asymptotic giant branch (AGB) undergoing helium shell burning. The more massive supergiants lose mass much more rapidly and all red supergiants appear to reach a similar mass of the order of 10 M☉ by the time their cores collapse. CaH-r vs. R-I diagrams are used to photometrically distinguish between red giants and dwarfs.

They will universally go on to burn heavier elements and undergo core-collapse resulting in a supernova.[22]. [26], Some red supergiants undergo blue loops where they temporarily increase in temperature before returning to the red supergiant state.



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