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 (i.e. nuclear fusion of hydrogen). 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 (M☉)) or a Herbig Ae/Be star (2–8 M☉). More massive (>8 M☉) 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 fusing and they are main-sequence objects.
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 a pre-main-sequence star. Pre-main-sequence stars become optically visible after reaching the stellar birthline. Pre-main-sequence stage will last less than 1 percent of a star's life (in contrast, the star will spend about 80 percent of its life on the main sequence).