Star are classified by the relative intensity of certain absorption lines occurring in their spectra, which is directly related to the surface temperature of the star. The stages of spectrum sequence, hottest to coldest, are designated by the letters: W, O, B, A, F, G, K, M and C. The numbers 0 to 9 are used to denote gradients in the pattern within each class. In addition there are neutron stars which are given a different classification.
Wolf-Rayet stars. Mostly He. up to 70,000 K
Lines of Hydrogen, Helium (HeI, HeII)
Hydrogen and intense HeI lines. HeII lines absent.
Hydrogen dominant lines. Calcium (CaII) lines. HeI and HeII lines absent.
Hydrogen and metals lines (CaII, Fe).
Hydrogen and metal lines. Many have notabe Iron lines. Some have molecular species.
Hydrogen lines, strong calcium lines and metal lines. Less intense violet light.
Metal lines. Molecular species notably Titanium oxide.
Metal lines. Molecular species notably C2.
Neutron stars and pulsars
A neutron star is a large mass with a gravitational field so high that the atoms itself have collapsed to neutrons. Pulsars are neutron stars that emit high levels of radiation with regular pulsation periods
A majority of stars are members of binary or multiple-star systems. Related stars rotate about a common centripetal point but are still far apart. A binary star system is two related stars. A cluster is a group of related stars. A double is a two unrelated stars which appear very close together. A triple is three unrelated stars that appear very close together.
The luminosity of a star can vary over time, either periodically or not. Stars evolve. They change from one star type to another over their lifetime.