page_title: Stellar classification
section_title: Yerkes spectral classification
hierarchical_section_title: Stellar classification / Modern classification / Yerkes spectral classification
caption_attribution_description: English: Spectra for dwarfs (luminosity class V) for standard spectral types taken from Pickles (1998).[1] Each row corresponds to one spectrum and the intensity to the brightness at that wavelength, ranging mostly over the optical. Created myself using the data from the paper and Matplotlib.
context_page_description: In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences. The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature.
  Most stars are currently classified under the Morgan–Keenan system using the letters O, B, A, F, G, K, and M, a sequence from the hottest to the coolest. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest. The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars.
context_section_description: 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 just surface temperature). Later, in 1953, after some revisions of list of standard stars and classification criteria, the scheme was named the Morgan–Keenan classification, or MK, and this system remains in use.
  Denser stars with higher surface gravity exhibit greater pressure broadening of spectral lines. The gravity, and hence the pressure, on the surface of a giant star is much lower than for a dwarf star because the radius of the giant is much greater than a dwarf of similar mass. Therefore, differences in the spectrum can be interpreted as luminosity effects and a luminosity class can be assigned purely from examination of the spectrum.
  A number of different luminosity classes are distinguished, as listed in the table below.
  Marginal cases are allowed; for example, a star may be either a supergiant or a bright giant, or may be in between the subgiant and main-sequence classifications. 
  In these cases, two special symbols are used:
  A slash (/) means that a star is either one class or the other.
  A dash (-) means that the star is in between the two classes.
  For example, a star classified as A3-4III/IV would be in between spectral types A3 and A4, while being either a giant star or a subgiant.
  Sub-dwarf classes have also been used: VI for sub-dwarfs (stars slightly less luminous than the main sequence).
  Nominal luminosity class VII (and sometimes higher numerals) is now rarely used for white dwarf or "hot sub-dwarf" classes, since the temperature-letters of the main sequence and giant stars no longer apply to white dwarfs.
  Occasionally, letters a and b are applied to luminosity classes other than supergiants; for example, a giant star slightly more luminous than typical may be given a luminosity class of IIIb.
  A sample of extreme V stars with strong absorption in He II λ4686 spectral lines have been given the Vz designation. An example star is HD 93129 B.
caption: Montage of false color spectra for main sequence stars[14]
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