Красные сверхгиганты: характеристики и особенности

Stars with a supergiant luminosity class with a spectral type of K or M

Red supergiants (RSGs) are stars with a supergiant luminosity class (Yerkes class I) and a stellar classification K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares A are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.

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|+ Temperature scale for red supergiants They are typically several hundred to over a thousand times the radius of the Sun, Main sequence stars more massive than about do not expand and cool to become red supergiants. Red supergiants at the upper end of the possible mass and luminosity range are the largest known. Their low surface gravities and high luminosities cause extreme mass loss, millions of times higher than the Sun, producing observable nebulae surrounding the star. By the end of their lives red supergiants may have lost a substantial fraction of their initial mass. The more massive supergiants lose mass much more rapidly and all red supergiants appear to reach a similar mass of the order of by the time their cores collapse. The exact value depends on the initial chemical makeup of the star and its rotation rate. Statistical analysis of many known variable red supergiants shows a number of likely causes for variation: just a few stars show large amplitudes and strong noise indicating variability at many frequencies, thought to indicate powerful stellar winds that occur towards the end of the life of a red supergiant; more common are simultaneous radial mode variations over a few hundred days and probably non radial mode variations over a few thousand days; only a few stars appear to be truly irregular, with small amplitudes, likely due to photospheric granulation. Red supergiant photospheres contain a relatively small number of very large convection cells compared to stars like the Sun. This causes variations in surface brightness that can lead to visible brightness variations as the star rotates. The spectra of red supergiants are similar to other cool stars, dominated by a forest of absorption lines of metals and molecular bands. Some of these features are used to determine the luminosity class, for example certain near infrared cyanogen band strengths and the Ca II triplet. Maser emission is common from the circumstellar material around red supergiants. Most commonly this arises from H2O and SiO, but hydroxyl (OH) emission also occurs from narrow regions. In addition to high resolution mapping of the circumstellar material around red supergiants, VLBI or VLBA observations of masers can be used to derive accurate parallaxes and distances to their sources. 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. Surface abundances of red supergiants are dominated by hydrogen even though hydrogen at the core has been completely consumed. In the latest stages of mass loss, before a star explodes, surface helium may become enriched to levels comparable with hydrogen. In theoretical extreme mass loss models, sufficient hydrogen may be lost that helium becomes the most abundant element at the surface. 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. Carbon and oxygen are quickly depleted and nitrogen enhanced as a result of the dredge up of CNO processed material from the fusion layers. Red supergiants are observed to rotate slowly or very slowly. Models indicate that even rapidly rotating main sequence stars should be braked by their mass loss so that red supergiants hardly rotate at all. Those red supergiants such as Betelgeuse that do have modest rates of rotation may have acquired it after reaching the red supergiant stage, perhaps through binary interaction. The cores of red supergiants are still rotating and the differential rotation rate can be very large. Less massive stars may develop a supergiant spectral luminosity class at relatively low luminosity, around when they are on the asymptotic giant branch (AGB) undergoing helium shell burning. 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 differently, usually producing a planetary nebula and white dwarf. Most AGB stars will not become supernovae although there is interest in a class of super AGB stars, those almost massive enough to undergo full carbon fusion, which may produce peculiar supernovae although without ever developing an iron core.]] Red supergiants develop from main sequence stars with masses between about and 30 or AGB stars may develop spectra with a supergiant luminosity class as they expand to extreme dimensions relative to their small mass, and they may reach luminosities tens of thousands times the sun's. Intermediate "super AGB" stars, around , can undergo carbon fusion and may produce an electron capture supernova through the collapse of an oxygen neon core. Main sequence stars, burning hydrogen in their cores, with masses between will have temperatures between about 25,000K and 32,000K and spectral types of early B, possibly very late O. They are already very luminous stars of due to rapid CNO cycle fusion of hydrogen and they have fully convective cores. In contrast to the Sun, the outer layers of these hot main sequence stars are not convective. Some red supergiants undergo blue loops where they temporarily increase in temperature before returning to the red supergiant state. This depends on the mass, rate of rotation, and chemical makeup of the star. While many red supergiants will not experience a blue loop, some can have several. Temperatures can reach 10,000K at the peak of the blue loop. 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. All red supergiants will exhaust the helium in their cores within one or two million years and then start to burn carbon. This continues with fusion of heavier elements until an iron core builds up, which then inevitably collapses to produce a supernova. The time from the onset of carbon fusion until the core collapse is no more than a few thousand years. In most cases, core collapse occurs while the star is still a red supergiant, the large remaining hydrogen rich atmosphere is ejected, and this produces a type II supernova spectrum. The opacity of this ejected hydrogen decreases as it cools and this causes an extended delay to the drop in brightness after the initial supernova peak, the characteristic of a Type II P supernova. The observed progenitors of type II P supernovae all have temperatures between 3,500K and 4,400K and luminosities between and This matches the expected parameters of lower mass red supergiants. A small number of progenitors of type II L and type IIb supernovae have been observed, all having luminosities around and somewhat higher temperatures up to 6,000K. These are a good match for slightly higher mass red supergiants with high mass loss rates. There are no known supernova progenitors corresponding to the most luminous red supergiants, and it is expected that these evolve to Wolf Rayet stars before exploding. Until the 21st century the largest number of red supergiants known in a single cluster was five in NGC 7419. Most red supergiants are found singly, for example Betelgeuse in the Orion OB1 association and Antares in the Scorpius–Centaurus association. Since 2006, a series of massive clusters have been identified near the base of the Crux Scutum Arm of the galaxy, each containing multiple red supergiants. RSGC1 contains at least 12 red supergiants, RSGC2 (also known as Stephenson 2) contains at least 26, RSGC3 contains at least 8, and RSGC4 (also known as Alicante 8) also contains at least 8. A total of 80 confirmed red supergiants have been identified within a small area of the sky in the direction of these clusters. These four clusters appear to be part of a massive burst of star formation 10–20 million years ago at the near end of the bar at the centre of the galaxy. Similar massive clusters have been found near the far end of the galactic bar, but not such large numbers of red supergiants.