- a narrow band in the Hertzsprung-Russell diagram in which 90 percent of all observed stars are plotted.
- a diagonal band on the Hertzsprung Russell diagram containing about 90% of all known stars; stars evolve onto and then off the band during their lifetime
- (as modifier)a main-sequence star
- The continuous, generally diagonal line or band in the Hertzsprung-Russell diagram ranging from the upper left to the lower right and representing stars of average size whose luminosities correspond predictably to their surface temperatures. Stars in this grouping maintain a stable nuclear reaction and experience only small fluctuations in luminosity and temperature. Main-sequence stars are believed to be in the stable, middle phase of their development; they are expected to move off the main sequence once the hydrogen in their core is exhausted. At that point, depending on its size, a main-sequence star will become a giant star, a supergiant star, or a white dwarf. The more massive the star, the faster it burns its nuclear fuel and the shorter it remains in the main sequence. See more at Hertzsprung-Russell diagram star.