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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. A protostar grows by accretion, acquiring mass from its surrounding envelope of interstellar dust and gas. By the time it is visible, the main accretion phase has ended and it has acquired virtually all of its mass but has not yet started hydrogen burning. The end of the main accretion phase to the start of hydrogen burning (i.e. zero age main sequence) is the pre-main sequence stage. A PMS star 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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