pre-main-sequence star

A pre-main-sequence star (PMS or pre-MS or PMS object) is a star in an early stage, specifically, before it has become a main sequence star (before its hydrogen burning). At this early point, the star's energy is from gravitational contraction (Kelvin-Helmholtz mechanism). Usage of terms vary but the term PMS generally indicates a phase that is distinct from and subsequent to the phase termed protostar. The PMS phase is after the bulk of the accretion that gave the star its mass, and after the star was directly embedded in the nebula that formed it. The phase includes the Hayashi track and possibly the Henyey track, and the stars develop accretion disks. For early stars, the PMS phase can be so short that it is hardly considered a phase, i.e., hydrogen burning develops very rapidly. Some PMS types:

• T-Tauri star (TTS).
• FU Orionis star (FUor).
• Herbig AeBe star (HAeBe), or more specifically, a Herbig Ae star (HAe) or Herbig Be star (HBe).
• UX Orionis variable (UXOR), a Herbig Ae/Be star that shows variability seeming to be temporarily obscured by dust.