Supergiant

Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram, with absolute visual magnitudes between about −3 and −8. The temperatures of supergiant stars range from about 3,400 K to over 20,000 K.

Definition

The title supergiant, as applied to a star, does not have a single concrete definition. The term giant star was first coined by Ejnar Hertzsprung when it became apparent that the majority of stars fell into two distinct regions of the Hertzsprung–Russell diagram. One region contained larger and more luminous stars of spectral types A to M, which received the name giant. 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.
Supergiants with spectral classes of O to A are typically referred to as blue supergiants, supergiants with spectral classes F and G are referred to as yellow supergiants, while those of spectral classes K to M are red supergiants. Another convention uses temperature: Supergiants with effective temperatures below 4800 K are deemed red supergiants; those with temperatures between 4800 and 7500 K are yellow supergiants, and those with temperatures exceeding 7500 K are blue supergiants. These correspond approximately to spectral types M and K for red supergiants, G, F, and late A for yellow supergiants, and early A, B, and O for blue supergiants.

Spectral luminosity class

Supergiant stars can be identified on the basis of their spectra, with distinctive lines sensitive to high luminosity and low surface gravity. In 1897, Antonia C. Maury had divided stars based on the widths of their spectral lines, with her class "c" identifying stars with the narrowest lines. Although it was not known at the time, these were the most luminous stars. In 1943, Morgan and Keenan formalised the definition of spectral luminosity classes, with class I referring to supergiant stars. The same system of MK luminosity classes is still used today, with refinements based on the increased resolution of modern spectra. Supergiants occur in every spectral class, from young blue class O supergiants to highly evolved red class M supergiants. Because they are enlarged compared with main-sequence and giant stars of the same spectral type, they have lower surface gravities, and changes can be observed in their line profiles. Supergiants are also evolved stars with higher levels of heavy elements than main-sequence stars. This is the basis of the MK luminosity system, which assigns stars to luminosity classes purely from observations of their spectra.
In addition to the line changes due to low surface gravity and fusion products, the most luminous stars have high mass-loss rates and resulting clouds of expelled circumstellar materials, which can produce emission lines, P Cygni profiles, or forbidden lines. The MK system assigns stars to luminosity classes: Ib for supergiants; Ia for luminous supergiants; and 0 or Ia⁺ for hypergiants. In reality there is much more of a continuum than well-defined bands for these classifications, and classifications such as Iab are used for intermediate-luminosity supergiants. Supergiant spectra are frequently annotated to indicate spectral peculiarities, for example B2 Iae or F5 Ipec.

Evolutionary supergiants

Supergiants can also be defined by a specific phase in the evolutionary history of certain stars. Stars with initial masses above quickly and smoothly initiate helium-core fusion after they have exhausted their hydrogen, and continue fusing heavier elements after helium exhaustion until they develop an iron core, at which point the core collapses to produce a Type II supernova. Once these massive stars leave the main sequence, their atmospheres inflate, and they are described as supergiants. Stars initially under 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. The phase where these stars have both hydrogen- and helium-burning shells is referred to as the asymptotic giant branch, as stars gradually become more and more luminous class M stars. Stars of may fuse sufficient carbon on the AGB to produce an oxygen-neon core and an electron-capture supernova, but astrophysicists categorise these as super-AGB stars rather than supergiants.

Categorisation of evolved stars

There are several categories of evolved stars that are not supergiants in evolutionary terms but may show supergiant spectral features or have luminosities comparable to supergiants.
Asymptotic-giant-branch 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, their being in a different stage of development, and their lives ending in a different way, astrophysicists prefer to keep them separate. The dividing line becomes blurred at around , where stars start to undergo limited fusion of elements heavier than helium. Specialists studying these stars often refer to them as super AGB stars, since they have many properties in common with AGB, such as thermal pulsing. Others describe them as low-mass supergiants since they start to burn elements heavier than helium and can explode as supernovae. Many post-AGB stars receive spectral types with supergiant luminosity classes. For example, RV Tauri has an Ia luminosity class despite being less massive than the Sun. Some AGB stars also receive a supergiant luminosity class, most notably W Virginis variables such as W Virginis itself, stars that are executing a blue loop triggered by thermal pulsing. 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. δ Cephei itself is an example, with a luminosity of and a mass of.
Wolf–Rayet stars are also high-mass luminous evolved stars, hotter, smaller, and visually less bright than most supergiants but often more luminous because of their high temperatures. 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. Just as the AGB stars occur in almost the same region of the HR diagram as red supergiants, Wolf–Rayet stars can occur in the same region of the HR diagram as the hottest blue supergiants and main-sequence stars.
The most massive and luminous main-sequence stars are almost indistinguishable from the supergiants they quickly evolve into. They have almost identical temperatures and very similar luminosities, and only the most detailed analyses can distinguish the spectral features that show they have evolved away from the narrow early O-type main-sequence to the nearby area of early O-type supergiants. Such early O-type supergiants share many features with WNLh Wolf–Rayet stars and are sometimes designated as slash stars, intermediates between the two types.
Luminous blue variables stars occur in the same region of the HR diagram as blue supergiants but are generally classified separately. They are evolved, expanded, massive, and luminous stars, often hypergiants, but they have a very specific spectral variability that defies assignment of a standard spectral type. LBVs observed only at a particular time, or over a period of time when they are stable, may simply be designated as hot supergiants or as candidate LBVs due to their luminosity.
Hypergiants are frequently treated as a different category of star from supergiants, although in all important respects they are just a more luminous category of supergiant. They are evolved, expanded, massive and luminous stars like supergiants, but at the most massive and luminous extreme, and with particular additional properties of undergoing high mass loss due to their extreme luminosities and instability. Generally only the more evolved supergiants show hypergiant properties, since their instability increases after high mass loss and some increase in luminosity.
Some B stars are supergiants, although other B stars are clearly not. Some researchers distinguish the B objects as separate from supergiants, while researchers prefer to define massive evolved B stars as a subgroup of supergiants. The latter has become more common, with the understanding that the B phenomenon arises separately in a number of distinct types of stars, including some that are clearly just a phase in the life of supergiants.

Properties

Supergiants have masses from 8 to 12 times the Sun upwards, and luminosities from about 1,000 to over a million times the Sun. They vary greatly in radius, usually from 30 to 500 or even in excess of 1,000 solar radii. 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. They go on to ignite successively heavier elements, usually all the way to iron. Also because of their high masses, they are destined to explode as supernovae.
The Stefan–Boltzmann law dictates that the relatively cool surfaces of red supergiants radiate much less energy per unit area than those of blue supergiants; thus, for a given luminosity, red supergiants are larger than their blue counterparts. Radiation pressure limits the largest cool supergiants to around 1,500 and the most massive hot supergiants to around a million . Stars near and occasionally beyond these limits become unstable, pulsate, and experience rapid mass loss.