pre-main-sequence star

A pre-main-sequence star (PMS or pre-MS or PMS object) is a star in an early stage after being a protostar and before becoming a main sequence star, in other words, before its hydrogen burning. At this early point, the star's energy is from gravitational contraction. Some types:

• T-Tauri star (TTS).
• FU Orionis star (FUor).
• Herbig Ae/Be star (HAeBe) or more specifically: 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.

Two distinctions used to classify pre-main-sequence stars and protostars are:

• whether it is still accreting material.
• whether it is still surrounded by a nebula.

Active pre-main-sequence stars often emit X-rays and ultraviolet rays.