Supergiant
Ejnar Hertzsprung first coined the term giant star when he noticed that most stars clustered into two distinct regions on the Hertzsprung, Russell diagram. One region held larger, more luminous stars of spectral types A to M. As astronomers realized some of these stars lacked measurable parallax yet were significantly larger and brighter than the bulk, the term super-giant emerged quickly as supergiant. This title does not have a single concrete definition today. Modern astronomy places supergiants in the top region of the Hertzsprung, Russell diagram with absolute visual magnitudes between about minus 3 and minus 8. Their temperatures range from approximately 3,400 K to over 20,000 K.
Antonia C. Maury divided stars based on spectral line widths in 1897, identifying class c as having the narrowest lines. These turned out to be the most luminous stars, though this was unknown at the time. Morgan and Keenan formalized the spectral luminosity classes in 1943, assigning class I to supergiant stars. This MK system remains in use today with refinements for modern spectra. Supergiants occur in every spectral class from young blue O-type to highly evolved red M-type. Because they are enlarged compared to main-sequence or giant stars of the same type, they possess lower surface gravities observable in their line profiles. The most luminous stars exhibit high mass-loss rates creating clouds of expelled circumstellar material that produce emission lines, P Cygni profiles, or forbidden lines. The MK system assigns Ib for supergiants, Ia for luminous supergiants, and 0 or Ia+ for hypergiants.
Stars with initial masses above eight solar masses quickly initiate helium-core fusion after exhausting hydrogen. They continue fusing heavier elements until developing an iron core that collapses into a Type II supernova. Once these massive stars leave the main sequence, their atmospheres inflate and become supergiants. Stars initially under eight solar masses never form an iron core and do not become supergiants in evolutionary terms. They eventually lose outer layers leaving white dwarf cores. Stars around eight solar masses may fuse sufficient carbon on the asymptotic giant branch to produce oxygen-neon cores and electron-capture supernovae. Astrophysicists categorize these as super-AGB stars rather than true supergiants. Asymptotic-giant-branch stars are highly evolved lower-mass red giants with luminosities comparable to more massive red supergiants but ending differently as planetary nebulae and white dwarfs.
Supergiants possess masses from 8 to 12 times the Sun upwards and luminosities ranging from about 1,000 to over a million times the Sun. Their radii vary greatly usually between 30 and 500 or even exceeding 1,000 solar radii. The Stefan, Boltzmann law dictates that relatively cool surfaces of red supergiants radiate much less energy per unit area than 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 solar radii and the most massive hot supergiants to around a million solar radii. Stars near and occasionally beyond these limits become unstable pulsating and experiencing rapid mass loss. Supergiants typically have surface gravities of around log(g) 2.0 cgs and lower though bright giants share statistically similar values. Cool luminous supergiants have lower surface gravities with the most luminous stars having log(g) around zero.
The abundance of various elements at the surfaces of supergiants differs significantly from less luminous stars. Supergiants may undergo convection of fusion products to the surface during their evolution. Cool supergiants show enhanced helium and nitrogen caused by convection of these fusion products during the main sequence of very massive stars. Helium forms in the core and shell by hydrogen fusion while nitrogen accumulates relative to carbon and oxygen during CNO cycle fusion. Red supergiants can be distinguished from luminous but less massive AGB stars by unusual chemicals including enhancement of carbon-13 lithium and s-process elements. Hotter supergiants show differing levels of nitrogen enrichment due to different mixing levels on the main sequence or because some blue supergiants are newly evolved from the main sequence. Post-red-supergiant stars generally display higher nitrogen relative to carbon due to convection of CNO-processed material to the surface and complete loss of outer layers.
Most Type II supernova progenitors are thought to be red supergiants while less common Type Ib/c supernovae come from hotter Wolf, Rayet stars that have completely lost more of their hydrogen atmosphere. Stars large enough to start fusing elements heavier than helium do not seem to have any way to lose enough mass to avoid catastrophic core collapse. The simple onion models showing red supergiants inevitably developing to an iron core and then exploding have been shown to be too simplistic. The progenitor for the unusual Type II Supernova 1987A was a blue supergiant thought to have already passed through the red supergiant phase. Much research now focuses on how blue supergiants can explode as supernovae and when red supergiants can survive to become hotter stars again. Almost by definition supergiants are destined to end their lives violently leaving behind neutron star or black hole remnants usually after a core-collapse supernova explosion.
Rigel stands as the brightest star in the constellation Orion representing a typical blue-white supergiant. The three stars forming Orion's Belt are all blue supergiants including Deneb which is the brightest star in Cygnus. Delta Cephei serves as the prototype for Classical Cepheid variables and yellow supergiants alongside Polaris. Antares and VV Cephei A stand out as red supergiants visible to the naked eye. Garnet Star is considered a red hypergiant due to its large luminosity making it one of the reddest stars visible to the naked eye. Rho Cassiopeiae is a variable yellow hypergiant among the most luminous naked-eye stars known. Betelgeuse remains a red supergiant that may have been a yellow supergiant in antiquity and is the second-brightest star in the constellation Orion.
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Common questions
What is the definition of a supergiant star?
Modern astronomy places supergiants in the top region of the Hertzsprung Russell diagram with absolute visual magnitudes between about negative 3 and negative 8. Their temperatures range from approximately 3,400 K to over 20,000 K.
When did astronomers first classify supergiant stars?
Antonia C. Maury divided stars based on spectral line widths in 1897 identifying class c as having the narrowest lines. Morgan and Keenan formalized the spectral luminosity classes in 1943 assigning class I to supergiant stars.
How do supergiant stars end their lives?
Supergiants are destined to end their lives violently leaving behind neutron star or black hole remnants usually after a core collapse supernova explosion. Stars with initial masses above eight solar masses quickly initiate helium core fusion until developing an iron core that collapses into a Type II supernova.
Which famous stars are examples of blue supergiants?
Rigel stands as the brightest star in the constellation Orion representing a typical blue white supergiant. The three stars forming Orion's Belt are all blue supergiants including Deneb which is the brightest star in Cygnus.
What distinguishes red supergiants from other evolved stars?
Red supergiants can be distinguished from luminous but less massive AGB stars by unusual chemicals including enhancement of carbon-13 lithium and s process elements. Cool supergiants show enhanced helium and nitrogen caused by convection of these fusion products during the main sequence of very massive stars.