Pre-main-sequence star

An observed PMS object can either be a T Tauri star, if it has fewer than 2 solar masses (M_☉), or else a Herbig Ae/Be star, if it has 2 to 8 M_☉. Yet more massive stars may have no observable pre-main-sequence stage because they contract too quickly as protostars and are surrounded by circumstellar gas. By the time they become visible, the hydrogen in their centers is already fusing and they are main-sequence objects. The creation of large reflecting telescopes has made it possible to study massive young stellar objects (YSOs), which include protostars and PMS stars.

The energy source of PMS objects is gravitational contraction and deuterium fusion, as opposed to hydrogen burning in main-sequence stars. In the Hertzsprung–Russell diagram, pre-main-sequence stars with more than 0.5 M_☉ first move vertically downward along Hayashi tracks as protostars, then leftward and horizontally along Henyey tracks until they finally halt at the main sequence. Pre-main-sequence stars with less than 0.5 M_☉ contract vertically along the Hayashi track for their entire evolution.

PMS stars can be differentiated empirically from main-sequence stars by using stellar spectra to measure their surface gravity. A PMS object has a larger radius than a main-sequence star with the same stellar mass and thus has a lower surface gravity. Although they are optically visible, PMS objects are rare relative to those on the main sequence, because their contraction lasts for only 1 percent of the lifetime of hydrogen fusion. During the early portion of the PMS stage, most stars have circumstellar disks, which are the sites of planet formation.