premainsequence

Premainsequence, or pre-main-sequence, is the evolutionary stage of stars before stable hydrogen fusion in the core. Stars form in molecular clouds and, after a protostellar phase, contract and heat up. Many PMS stars continue to accrete from circumstellar disks and display outflows. Observational representatives include T Tauri stars (low-mass) and Herbig Ae/Be stars (intermediate-mass).

During contraction, the internal structure changes. Very low-mass PMS stars are largely convective and follow the Hayashi track on the Hertzsprung-Russell diagram, moving downward at roughly constant temperature. As contraction proceeds, a radiative core develops and the track shifts toward higher temperature along the Henyey track until hydrogen fusion begins on the main sequence. Early deuterium burning can provide additional luminosity and influence evolution; accretion can also affect brightness.

Timescales and observations. The PMS phase lasts from a few hundred thousand to tens of millions of years depending on mass: high-mass stars reach the main sequence quickly, while low-mass stars take longer. PMS stars are common in star-forming regions and show emission lines such as H-alpha, infrared excess from disks, and sometimes jets. Their disks are the sites of planet formation and dissipate over time.

End state. The premainsequence ends when stable core hydrogen burning begins, and the star settles on the main sequence. The PMS phase links star formation with main-sequence evolution and informs theories of early stellar physics and planet formation.