<P> Accretion of material onto the protostar continues partially from the newly formed circumstellar disc . When the density and temperature are high enough, deuterium fusion begins, and the outward pressure of the resultant radiation slows (but does not stop) the collapse . Material comprising the cloud continues to "rain" onto the protostar . In this stage bipolar jets are produced called Herbig - Haro objects . This is probably the means by which excess angular momentum of the infalling material is expelled, allowing the star to continue to form . </P> <P> When the surrounding gas and dust envelope disperses and accretion process stops, the star is considered a pre--main sequence star (PMS star). The energy source of these objects is gravitational contraction, as opposed to hydrogen burning in main sequence stars . The PMS star follows a Hayashi track on the Hertzsprung--Russell (H--R) diagram . The contraction will proceed until the Hayashi limit is reached, and thereafter contraction will continue on a Kelvin--Helmholtz timescale with the temperature remaining stable . Stars with less than 0.5 M thereafter join the main sequence . For more massive PMS stars, at the end of the Hayashi track they will slowly collapse in near hydrostatic equilibrium, following the Henyey track . </P> <P> Finally, hydrogen begins to fuse in the core of the star, and the rest of the enveloping material is cleared away . This ends the protostellar phase and begins the star's main sequence phase on the H--R diagram . </P> <P> The stages of the process are well defined in stars with masses around 1 M or less . In high mass stars, the length of the star formation process is comparable to the other timescales of their evolution, much shorter, and the process is not so well defined . The later evolution of stars are studied in stellar evolution . </P>

Where does most star formation occur in milky way