Supergiant
[1] Subsequently, as they lacked any measurable parallax, it became apparent that some of these stars were significantly larger and more luminous than the bulk, and the term super-giant arose, quickly adopted as supergiant.
Supergiant stars can be identified on the basis of their spectra, with distinctive lines sensitive to high luminosity and low surface gravity.
[15] The same system of MK luminosity classes is still used today, with refinements based on the increased resolution of modern spectra.
Because they are enlarged compared to main-sequence and giant stars of the same spectral type, they have lower surface gravities, and changes can be observed in their line profiles.
Stars initially under 10 M☉ will never form an iron core and in evolutionary terms do not become supergiants, although they can reach luminosities thousands of times the sun's.
They cannot fuse carbon and heavier elements after the helium is exhausted, so they eventually just lose their outer layers, leaving the core of a white dwarf.
Asymptotic-giant-branch (AGB) and post-AGB stars are highly evolved lower-mass red giants with luminosities that can be comparable to more massive red supergiants, but because of their low mass, being in a different stage of development (helium shell burning), and their lives ending in a different way (planetary nebula and white dwarf rather than supernova), astrophysicists prefer to keep them separate.
The dividing line becomes blurred at around 7–10 M☉ (or as high as 12 M☉ in some models[18]) where stars start to undergo limited fusion of elements heavier than helium.
A very small number of Mira variables and other late AGB stars have supergiant luminosity classes, for example α Herculis.
Classical Cepheid variables typically have supergiant luminosity classes, although only the most luminous and massive will actually go on to develop an iron core.
The majority of them are intermediate mass stars fusing helium in their cores and will eventually transition to the asymptotic giant branch.
They have spectra dominated by helium and other heavier elements, usually showing little or no hydrogen, which is a clue to their nature as stars even more evolved than supergiants.
The latter has become more common with the understanding that the B[e] phenomenon arises separately in a number of distinct types of stars, including some that are clearly just a phase in the life of supergiants.
They are massive enough to begin helium-core burning gently before the core becomes degenerate, without a flash and without the strong dredge-ups that lower-mass stars experience.
The supergiant luminosity class is assigned on the basis of spectral features that are largely a measure of surface gravity, although such stars are also affected by other properties such as microturbulence.
A much smaller grouping consists of very low-luminosity G-type supergiants, intermediate mass stars burning helium in their cores before reaching the asymptotic giant branch.
A distinct grouping is made up of high-luminosity supergiants at early B (B0-2) and very late O (O9.5), more common even than main sequence stars of those spectral types.
[24] The supergiants lie more or less on a horizontal band occupying the entire upper portion of the HR diagram, but there are some variations at different spectral types.
This means that hot supergiants lie on a relatively narrow band above bright main sequence stars.
They are not classified separately into normal (Ib) and luminous (Ia) supergiants, although they commonly have other spectral type modifiers such as "f" for nitrogen and helium emission (e.g. O2 If for HD 93129A).
Stars that would be brighter than this shed their outer layers so rapidly that they remain hot supergiants after they leave the main sequence.
[26] Red supergiants can be distinguished from luminous but less massive AGB stars by unusual chemicals at the surface, enhancement of carbon from deep third dredge-ups, as well as carbon-13, lithium and s-process elements.
Post-red supergiant stars have a generally higher level of nitrogen relative to carbon due to convection of CNO-processed material to the surface and the complete loss of the outer layers.
Unlike lower-mass stars, however, they begin to fuse helium in the core smoothly and not long after exhausting their hydrogen.
This means that they do not increase their luminosity as dramatically as lower-mass stars, and they progress nearly horizontally across the HR diagram to become red supergiants.
They cannot lose enough mass to form a white dwarf, so they will leave behind a neutron star or black hole remnant, usually after a core collapse supernova explosion.
Part of the theorized population III of stars, their existence is necessary to explain observations of elements other than hydrogen and helium in quasars.
Possibly larger and more luminous than any supergiant known today, their structure was quite different, with reduced convection and less mass loss.
The simple "onion" models showing red supergiants inevitably developing to an iron core and then exploding have been shown, however, to be too simplistic.
μ Cephei is considered a red hypergiant due to its large luminosity and it is one of the reddest stars visible to the naked eye and one of the largest in the galaxy.
