Stellar Classification - Yerkes Spectral Classification

The Yerkes spectral classification, also called the MKK system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan, Philip C. Keenan, and Edith Kellman from Yerkes Observatory. This two-dimensional (temperature and luminosity) classification scheme is based on spectral lines sensitive to stellar temperature and surface gravity which is related to luminosity (whilst the Harvard classification is based on surface temperature only). Later, in 1953, after some revisions of list of standard stars and classification criteria, the scheme was named MK (by William Wilson Morgan and Phillip C. Keenan initials).

Since the radius of a giant star is much greater than a dwarf star while their masses are roughly comparable, the gravity and thus the gas density and pressure on the surface of a giant star are much lower than for a dwarf. These differences manifest themselves in the form of luminosity effects which affect both the width and the intensity of spectral lines which can then be measured. Denser stars with higher surface gravity will exhibit greater pressure broadening of spectral lines.

A number of different luminosity classes are distinguished:

• 0 hypergiants
• I supergiants
  • Ia-0 (hypergiants or extremely luminous supergiants (later addition)), Example: Eta Carinae (spectrum-peculiar)
  • Ia (luminous supergiants), Example: Deneb (spectrum is A2Ia)
  • Iab (intermediate luminous supergiants) Example: Betelgeuse (spectrum is M2Iab)
  • Ib (less luminous supergiants)
• II bright giants
  • IIa, Example: β Scuti (HD 173764) (spectrum is G4 IIa)
  • IIab Example: HR 8752 (spectrum is G0Iab:)
  • IIb, Example: HR 6902 (spectrum is G9 IIb)
• III normal giants
  • IIIa, Example: ρ Persei (spectrum is M4 IIIa)
  • IIIab Example: δ Reticuli (spectrum is M2 IIIab)
  • IIIb, Example: Pollux (spectrum is K2 IIIb)
• IV subgiants
  • IVa, Example: ε Reticuli (spectrum is K1-2 IVa-III)
  • IVab
  • IVb, Example: HR 672 A (spectrum is G0.5 IVb)
• V main-sequence stars (dwarfs)
  • Va, Example: AD Leonis (spectrum M4Vae)
  • Vab
  • Vb, Example: 85 Pegasi A (spectrum G5 Vb)
  • "Vz", Example: LH10 : 3102 (spectrum O7 Vz), located in the Large Magellanic Cloud.
• VI subdwarfs. Subdwarfs are generally represented with a prescript of sd or esd (extreme subdwarf) in front of the spectra.
  • sd, Example: SSSPM J1930-4311 (spectrum sdM7)
  • esd, Example: APMPM J0559-2903 (spectrum esdM7)
• VII (uncommon) white dwarfs. White dwarfs are represented with a prescript wD or WD.

Marginal cases are allowed; for instance a star classified as Ia-0 would be a very luminous supergiant, verging on hypergiant. Examples are below. The spectral type of the star is not a factor.