Table: Luminosity Classes: The main bands on the Hertzsprung-Russell (HR) diagram of various kinds of stars are the luminosity classes. Note that a luminosity class is NOT a band of the same luminosity: luminosities can vary quite widely within a luminosity class. Note also, there are actually NO gaps between the bands, except that white dwarfs form a rather isolated band:

0 := hypergiants: See Wikipedia: List of most massive stars, where the current record most massive star is ∼ 300 M_☉.
Ia := more luminous supergiants
Ib := less luminous supergiants II := bright giants
III := giants
IV := subgiants
V := main-sequence stars: These are also called dwarf stars, but that name now seems a disfavored term,


except that main-sequence K stars and M stars are called, respectively, K dwarfs and M dwarfs, and collectively red dwarfs. The main sequence extends down to brown dwarfs (divided into L dwarfs, T dwarfs, and Y dwarfs) which are NOT considered stars at all in that they never have significant nuclear burning.


VI := subdwarf stars: They are main-sequence stars that lie a bit below


the ordinary main-sequence stars because of low metallicity. Their chemical abundance ratio [Fe/H] <∼ 0.3. Note chemical abundance ratio [Fe/H] is a proxy for metallicity Z. They are probably all Population II stars with ages >∼ 12 Gyr and Z <∼ 0.004 (see Table: Stellar Population Metallicity for the Milky Way). Recall the age of the observable universe = 13.797(23) Gyr (Planck 2018), and so subdwarf stars formed relatively near the time of the Big Bang.

VII := white dwarfs


Local file: local link: luminosity_class_table.html.
File: Star file: luminosity_class_table.html.