A dwarf star is a term used, oddly enough, to describe any star that is of normal size for its mass. Such stars lie on the main sequence and are thus in the process of converting hydrogen to helium by nuclear fusion in their cores. The Sun, for example, is classified as a yellow dwarf. Red dwarfs are the smallest and least luminous of main sequence stars

 

Highly evolved dwarf stars that have left the main sequence include white dwarfs, which are collapsed stars that are still hot and shining by virtue of stored thermal energy, and black dwarfs, which are become so cold that they no longer give off any visible radiatio. Brown dwarfs are substellar objects with insufficient mass to be able to fuse hydrogen in their cores.

 

Subdwarf

A subdwarf is a star that is smaller and less luminous by one to two magnitudes (2.5–6 times fainter) than a normal dwarf star of the same spectral type. Subdwarfs are mainly old, galactic halo population II objects of types F, G, and K. They are denoted by the prefix "sd", as in sdK7, and are placed in luminosity class VI.

 

Subluminous star

A subluminous star is a star, such as a white dwarf, subdwarf, or central star of a planetary nebula, that is fainter than a main sequence star of the same spectral type. Most subluminous stars are old (population II) objects.