A pre-main-sequence star (also known as a PMS star and PMS object) is a
star
A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...
in the stage when it has not yet reached the
main sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...
. Earlier in its life, the object is a
protostar that grows by acquiring mass from its surrounding envelope of interstellar dust and gas. After the
protostar blows away this envelope, it is optically visible, and appears on the
stellar birthline in the
Hertzsprung-Russell diagram. At this point, the star has acquired nearly all of its mass but has not yet started
hydrogen burning (i.e.
nuclear fusion of hydrogen). The star then contracts, its internal temperature rising until it begins hydrogen burning on the
zero age main sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...
. This period of contraction is the pre-main sequence stage.
An observed PMS object can either be a
T Tauri star, if it has fewer than 2
solar mass
The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass o ...
es (), or else a
Herbig Ae/Be star, if it has 2 to 8 . Yet more massive stars have no pre-main-sequence stage because they contract too quickly as protostars. By the time they become visible, the hydrogen in their centers is already fusing and they are
main-sequence objects.
The energy source of PMS objects is