A pre-main sequence star (PMS star, or PMS object) is a star in the stage when it has not yet reached the main sequence. It can be a T Tauri star or FU Orionis star (<2 solar mass) or an 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). On the HR diagram, the pre-main sequence stage of stars with masses>0.5 Solar mass translates into a move along Hayashi tracks (almost vertically down) and later along Henyey tracks (almost horizontally to the left, towards the main sequence).

PMS stars can be differentiated from dwarf stars by using stellar spectra to measure the correlation between gravity and temperature. A PMS star will be "puffier" than a main sequence star.

Until the surrounding matter is falling onto the central condensation, it is considered to be a protostar. When the surrounding gas/dust envelope disperses and accretion process stops, the star is considered as pre-main sequence star. 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 disks, probable sites of planet formation.

See also:

• Hertzsprung-Russell Diagram - University of Oregon
• Hertzsprung-Russell Diagram - Wikipedia.
• Main Sequence - Wikipedia.
• Main Sequence: Formation - Wikipedia.
• Protostar - Wikipedia.
• "Star and Planet Formation and Early Evolution" - NASA/JPL Planet Quest.

(Text-Source: "Pre-main sequence star" - Wikipedia.  Image-Source: "The T Tauri Star Forming System" - NASA-APOD)