Dwarf Star

Definition

Ejnar Hertzsprung and Henry Norris Russell independently found the existence of stars with the same spectral type but different luminosities. Since the luminosity satisfies the relationship L ~ R²T⁴, the most luminous ones must be larger than the dimmer ones and they coined the terms giant and dwarf stars. Based on the existing differences in the spectral features of stars with the same type, William W. Morgan and Phillip C. Keenan, from Yerkes Observatory, introduced the so-called “luminosity class” represented by a Roman number. One of these classes, the V one, was the Main-Sequence class, also known as dwarfs since normal stars at this stage have the minimum radius. However, since main-sequence blue stars have very large masses and radii, the term “dwarf star” is at present usually used to design main sequence stars with a mass equal or smaller than the Sun. For instance solar-like stars are yellow dwarfs, and main-sequence M stars are red dwarfs. The term dwarf is also...