- The Hertzsprung-Russell diagram relates the brightness of a star to its temperature.
- Zero-age Main Sequence
- The position, on the main sequence of a Hertzsprung-Russell diagram, reached by stars after they have passed through their T Tauri phase. It corresponds to a star with 75% hydrogen, 23% helium and 2% metals in its core. Nuclear fusion then gradually changes this ratio and the star evolves across the main sequence.
See also: Star.