Red supergiant
This system uses certain diagnostic spectral lines to estimate the surface gravity of a star, hence determining its size relative to its mass.
Exceptionally bright, low surface gravity, stars with strong indications of mass loss may be designated by luminosity class 0 (zero) although this is rarely seen.
They have spectral types of K and M, hence surface temperatures below 4,100 K.[9] They are typically several hundred to over a thousand times the radius of the Sun,[9] although size is not the primary factor in a star being designated as a supergiant.
Their low surface gravities and high luminosities cause extreme mass loss, millions of times higher than the Sun, producing observable nebulae surrounding the star.
[13] 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.
[14] 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.
[16] In addition to high resolution mapping of the circumstellar material around red supergiants,[17] VLBI or VLBA observations of masers can be used to derive accurate parallaxes and distances to their sources.
[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.
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.
Carbon and oxygen are quickly depleted and nitrogen enhanced as a result of the dredge-up of CNO-processed material from the fusion layers.
Intermediate "super-AGB" stars, around 7-9 M☉, can undergo carbon fusion and may produce an electron capture supernova through the collapse of an oxygen-neon core.
[24] Main-sequence stars, burning hydrogen in their cores, with masses between 10 and 30 or 40 M☉ 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 10,000–100,000 L☉ due to rapid CNO cycle fusion of hydrogen and they have fully convective cores.
[12] The supergiants continue to cool and most will rapidly pass through the Cepheid instability strip, although the most massive will spend a brief period as yellow hypergiants.
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.
This continues with fusion of heavier elements until an iron core builds up, which then inevitably collapses to produce a supernova.
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.
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.
