Dwarf star

(astronomy) A star that typically has surface temperature of 5730 K, radius of 428,000 miles (690,000 kilometers), mass of 2 × 10³³ grams, and luminosity of 4 × 10³³ ergs per second. Also known as main sequence star.

A star whose state of evolution resembles that of the Sun. The term “dwarf star” derives from the work of Ejnar Hertzsprung and Henry Norris Russell, who distinguished two kinds of stars, large ones called giants (or supergiants) and smaller ones called dwarfs. The term “dwarf” is synonymous with “main sequence star” (luminosity class V) and implies not so much size as evolutionary condition. Dwarfs are stars that, like the Sun, fuse hydrogen into helium in their cores, the thermonuclear reactions providing energy and support. Dwarfs range over the entire spectral sequence. At the cool limit (where spectral class M converts to L), they have effective temperatures around 2000 K (3100°F), absolute visual magnitudes of +20, and bolometric luminosities of about 10⁻⁴ times the solar luminosity. At the hot O3 limit, the values are respectively 55,000 K (100,000°F), −7, and over 10⁶ solar. The Sun, a G2 dwarf, falls in the middle at 5780 K (9950°F) and an absolute visual magnitude of +4.83. These properties are produced by a mass range from 0.08 solar mass for warm L dwarfs, below which full hydrogen fusion cannot be turned on, to over 100 solar masses at class O3. See also Giant star; Hertzsprung-Russell diagram; Magnitude (astronomy); Sun.

At the high-mass end, above about 10 solar masses, the dwarf stage lasts 2–20 million years, and O stars turn into supergiants and supernovae. At class G8 and a mass of 0.8 solar, the lifetime equals the age of the Milky Way Galaxy, and dwarfs between this limit and 10 solar masses become white dwarfs. See also Spectral type; Star; Stellar evolution; Stellar rotation; Supergiant star; Supernova; White dwarf star.