The Luminosity Classes

An important factor that can broaden spectral lines is pressure. With increasing pressure in the stars outer layers more and more atoms will be disturbed during the time when emitting or absorbing a photon. This results in a change of energy of the levels of the atom. Thus the width of a spectral line increases as pressure increases.

The pressure in the outer layers of a star can vary in a wide range.

The masses of the stars lie in the range from a quarter of a solar mass up to 20 solar masses but radii range from 0.01 to 1000 solar radii. Therefore the density and with it the pressure vary in a very wide range. A physically large star (e.g. Deneb) is usually brighter and will have narrower spectral lines than a smaller star (e.g. Vega).

So we have another component of the spectral classification, the Luminosity Class.

The luminosity class is added in Roman numerals after the temperature spectral class.
ISupergiants
IIBright Giants
IIIGiants
IVSubgiants
VDwarfs (=Main Sequence Stars)
VISubdwarfs
VIIWhite Dwarfs
Our sun is a normal main sequence star an is classified as G2V, Vega as A0V, Betelgeuze as M2vIb (v means variable).

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