The diagonal belt of stars on the Hertzsprung-Russell diagram stretching from the high temperature and luminosity region of the diagram down to the low temperature end: A star's position in the belt depends on its mass, and once on the Main Sequence, a star remains there for most of its existence.

The point on the Hertzsprung-Russell diagram occupied by a star that has a core temperature high enough for nuclear reactions: The Zero Age Main Sequence is a major group of stars on a Hertzsprung-Russell diagram by a practically uniform band of brightness and surface temperatures that become greater and greater.