Pre-Main-Sequence Stars

A pre-main-sequence star(PMS) is a star intermediate between a protostar and a main-sequence star. PMS stars with masses of less than 2 solar masses are known as T-Tauri Stars(an artist's impression of a T-Tauri Star is shown above as the top-most image). PMS stars with masses between 2-8 solar masses are known as Herbig Ae/Be stars. Any star above 8 solar masses doesn't go through a PMS phase because it gravitationally contracts so much as a protostar that it already starts to undergo nuclear fusion.

PMS stars under 3 Solar Masses follow the Hayashi track of the HR-Diagram vertically downward. This track, named after Japanese astrophysicist Chushiro Hayashi, is a sub-plot of luminosity against temperature for infant stars with masses less than 3 solar masses. PMS stars go down the track so they become dimmer and dimmer but maintain the same temperatures. Once they develop a radiative zone, where electromagnetic radiation is emitted, they enter the Henyey track or nuclear fusion begins which lets them enter the main sequence. If they enter the Henyey track, where the luminosity doesn't change as the star gets ready to start undergoing nuclear fusion. Once the process of nuclear fusion occurs within the core, the star is officially in the main sequence.