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A pre-main-sequence star (also known as a PMS star and 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 (fewer than 2 solar masses) or a Herbig Ae/Be star (2–8 solar masses). More massive (>8 Solar mass) stars in pre-main-sequence stage are not observed, because they evolve very quickly: when they become visible (i.e. disperses surrounding circumstellar gas and dust cloud), the hydrogen in the center is already burning and they are main-sequence objects.
The energy source of these objects is gravitational contraction (as opposed to hydrogen burning in main-sequence stars). On the Hertzsprung–Russell diagram, the pre-main-sequence stage of stars with masses more than 0.5 solar masses translates into a move along Hayashi tracks (almost vertically down) and later along Henyey tracks (almost horizontally to the left, towards the main sequence), while the pre-main-sequence stage of stars with masses less than 0.5 solar masses translates into a move along the Hayashi track for the entirety of their pre-main-sequence stage.
PMS stars can be differentiated from main-sequence dwarf stars by using stellar spectra to measure the correlation between gravity and temperature. A PMS star will have a larger radius than a main-sequence star, and thus be less dense and have lower surface gravity.
While 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.
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