Pre-main sequence star is a star in the stage when it has not yet reached the main sequence. It can be T Tauri star (<2 solar mass) or Herbig Ae/Be stars (2-8 solar mass).

The energy source of these objects is gravitational contraction (as opposed to hydrogen burning in main sequence stars). During pre-main sequence stage stars with masses>0.5 Solar mass on HR diagram at first moves along Hayashi tracks (almost vertically down), later - along Henyey tracks (almost horizontally to the left, towards the main sequence).

Pre-main sequence stars become optically visible after stellar birthline. Pre-main sequence stage will last less than 1% of a star's life (in contrast, the star will spend about 80% of its life on the main sequence).

It is believed that during this stage all stars have dense circumstellar discs, probable sites of planet formation.

Astronomy | Star types | Hertzsprung-Russell classifications