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Key Facts & Information

  • Supergiant stars are among the most massive and most luminous stars with masses ranging from 10 to 70 times greater than our Sun. These stars occupy the top region of the Hertzsprung-Russell diagram with estimated absolute magnitudes between -3 and -8. Their temperature range is around 3,450 K to 20,000 K and they have varying brightness with some being 30,000 times brighter than the Sun.
  • These stars are often seen in young cosmic structures such as open clusters, arms, or spiral galaxies and irregular galaxies.
  • Supergiants are more abundant in irregular galaxies and are not found in elliptical galaxies, or globular clusters, which are believed to be mainly composed of old stars.
  • The term giant star was first used by Hertzsprung when he noticed that the majority of the stars are on the two distinct regions of the Hertzsprung- Russell Diagram.
  • One includes larger and more luminous stars with spectral types A to M, and were named giant.
  • Due to their lack of any measurable parallax, it seemed that some of these stars were significantly larger and are more luminous, thus giving birth to the term super-giant or supergiant.

Spectral Luminosity Classes

  • Supergiant stars can be identified according to their spectra, with distinctive lines sensitive to low surface gravity and high luminosity.
  • In 1897, Antonia Maury divided the stars according to the widths of their spectral lines, with class “c” being the stars with the narrowest lines, which are actually the most luminous stars.
  • In 1943, Morgan and Keenan formalized the definition of the spectral luminosity classes, with class I referring to the supergiants.
  • Supergiant stars occur in every spectral class from the young blue class O supergiant stars to highly evolved as the red class M supergiant stars.
  • They have lower surface gravities, and changes can be observed in their line profiles due to their massiveness compared to the main-sequence and giant stars of the same type.
  • Supergiant stars are also evolved stars, with higher levels of heavy elements compared to the main-sequence stars.
  • The line changes, due to the low surface gravity and fusion products, result in the high mass loss rate and resulting clouds of expelled circumstellar materials producing emission line, P Cygni profiles, or forbidden lines.
  • The MK luminosity system assigns stars in their corresponding luminosity classes, in which Ib is for supergiants, Ia for luminous supergiants, and 0 or Ia+ for hypergiants.
  • Also, Iab is used for intermediate luminosity supergiants.

Properties

  • Supergiant stars have masses ranging from around 10 to 70 times greater than our Sun.
  • They have great variation in terms of radius, usually from 30 to 500, or sometimes even exceeding 1,000 solar radii.
  • Due to their huge masses, supergiants have short life spans ranging from around 10 to 50 million years. They burn very quickly through their hydrogen supplies, resulting in shorter life spans.
  • Supergiant stars have masses ranging from 8 to 12 solar masses and luminosities from around 1,000 times greater than our Sun.
  • These stars are massive enough to start helium-core burning gently before the core becomes degenerate, without a flash and without the strong dredge-ups that lower-mass stars experience.
  • The Stefan-Boltzmann law suggests that the relatively cool surfaces of red supergiant stars radiate significantly less amounts of energy per unit area than the blue supergiants.
  • Red supergiants also have higher luminosities than the blue ones.
  • However, blue supergiants are usually the hottest stars.
  • Supergiants also vary in temperature, mid-M class stars range from 3,000 to 3,400 K, while the hottest O class stars have temperatures above 40,000 K.
  • Among the classes O to M, the majority of the supergiants are of spectral type B.
  • Supergiants lie around the horizontal band, which occupies the entire upper part of the Hertzsprung-Russell diagram, although they vary at different spectra types.
  • These variations are partly due to the different methods in assigning the stars at their corresponding luminosity classes at different spectral types, and also due to the physical differences of the stars.
  • The bolometric luminosity of a star reflects the star’s total output of electromagnetic radiation at all wavelengths.
  • The bolometric luminosity of very hot and very cool stars is dramatically higher than the visual luminosity.
  • This bolometric correction is approximated to be around one magnitude for mid B, late K, and early M stars, increasing to three magnitudes for O and mid M stars.
  • Supergiants are larger and more luminous than the main-sequence stars with the same temperature.
  • Hot supergiant stars lie on a narrow and above the bright main sequence.
  • The instability strip in the Hertzsprung-Russell Diagram crosses the region of the superiants, thus many yellow supergiants are Classical Cepheid Variables.
  • Yellow supergiants are not massive stars, but are stars of intermediate mass with low surface gravities.

Evolved Stars

  • Supergiant stars can also be a specific phase in the evolutionary history of some stars.
  • Stars with initial masses above 8-10, initiate helium core fusion after exhausting their hydrogen, and continue fusing heavier elements until they develop an iron core, which collapses to produce Type 2 supernova.
  • The atmospheres of these stars inflate once they leave the main sequence, and are described as supergiants.
  • On the other hand, stars with initial masses under 10 will never form an iron core and do not become supergiants, even if they have luminosity a thousand times more than our Sun.
  • They cannot fuse hydrogen and heavier elements, thus losing their outer layers and leaving the core of a white dwarf.
  • Asymptotic Giant Branch (AGB) refers to the phase in which stars have both hydrogen and helium-burning shells, as stars gradually become more and more luminous class M stars.
  • Stars with initial masses of 8 to 10 may fuse sufficient carbon on the AGB, producing an oxygen-neon core and an electron-capture supernova, however, astrophysicists categorize these as super-AGB stars instead of supergiants.

Chemical Abundances

  • Supergiants’ abundance of various elements at their surface is different from less luminous stars. Supergiant stars are evolved stars and have undergone convection of fusion products to the surface.
  • Cool supergiants have enhanced helium and nitrogen at their surface due to the convection of these fusion products during the main sequence, to dredge-ups, shell burning, and to the loss of the outer layers of the star.
  • Helium is formed in the core and shell of the stars by the fusion of hydrogen and nitrogen, which accumulates relative to carbon and oxygen during the CNO cycle fusion, making carbon and oxygen abundances reduced.
  • Hotter supergiant stars have different levels of nitrogen enrichment, which may be due to the different levels of mixing on the sequence.
  • This is due to the convection CNO-processed material to the surface and the complete loss of outer layers.
  • Surface enhancement of helium is also stronger in post-red supergiants.

Red Supergiants

  • Red supergiant stars have a prevalence of around 0.001% with spectral types K and M.
  • These supergiants have temperatures ranging from around 3,500 to 4,500 K, and luminosities around 1,000 to 800,000 times than the Sun.
  • They are usually 10 to 40 times more massive than the Sun.
  • Red supergiants usually have a life span of 3 to 100 million years.
  • Their hydrogen supplies have already exhausted and their outer layers have expanded as they leave the main sequence.
  • Some red supergiant stars that can still create heavier elements explode as type-II supernovae.

Blue Supergiants

  • Blue supergiant stars are also rare like the red supergiant stars.
  • These supergiants are the hottest stars in the universe with temperatures usually ranging from 10,000 to 50,000 K or more, and with spectral types O, and B.
  • They have luminosities around 10,000 to 1 million times more than the Sun and have masses of around 20 to 1,000 solar masses.
  • Blue supergiants have a short life span of 10 million years or so.
  • They burn their hydrogen supplies quickly due to their massiveness.

Evolved Star Categories

  • There are several categories of evolved stars which are not supergiants, but may show supergiant spectral features of luminosities similar to supergiant stars.

Asymptotic-Giant-Branch

  • Asymptotic-giant-branch (AGB) and post-AGB stars are highly evolved lower-mass red giants with luminosities similar to red supergiants.
  • They are sometimes described as low-mass supergiants as they can burn heavier elements and can explode as supernovae.

Classical Cepheid Variables

  • Classical Cepheid variables usually have supergiant luminosity classes.
  • The most luminous and most massive classical cepheid variables can develop an iron core.
  • The majority of them are intermediate mass stars that fuse helium inside their cores and will transition to the asymptotic giant branch.

Wolf-Rayet Stars

  • Wolf-Rayet stars are high-mass luminous evolved stars and are smaller and hotter than most supergiants.
  • They are visually less bright, but usually more luminous due to their high temperatures.
  • These stars occur in the same region of the Hertzsprung-Russell diagram as the hottest blue supergiants and main-sequence stars.

Luminous Blue Variables

  • Luminous blue variables (LBVs) also occur in the same band of the HR diagram as the blue supergiants.
  • These stars are evolved, expanded, massive, and luminous, but they vary in specific spectral visibility.

Hypergiants

  • Hypergiants are usually treated as a different category of stars from the supergiant stars.
  • They are evolved, expanded, massive, and as luminous as the supergiants, but at the most massive and luminous extreme.
  • More evolved supergiants usually show hypergiant properties as their instability increases after the high mass loss.

Famous Examples

  • VY Canis Majoris, a red supergiant, is one of the largest stars discovered, which is 1,000 times bigger than our Sun.
  • Icarus, a blue supergiant, is the most distant star detected around 14 billion light years away from us.
  • Mu Cephei, a red supergiant, is one of the reddest stars visible to the naked eye and one of the largest in the galaxy.
  • Rigel, a blue supergiant, is the brightest in the constellation Orion
  • Antares, a red supergiant, is the brightest in the constellation Scorpio.
  • Deneb, a white supergiant, is the brightest star in the constellation Cygnus.
  • Delta Cephei, a yellow supergiant, is the famous prototype of Cepheid Variable.