From Protostar to the Main Sequence

As the temperature in the centre of the protostar rises so does the pressure of the gas in the centre. Eventually this pressure will become high enough to stop the centre collapsing any more. The ultimate maximum temperature reached will depend on the initial mass of the clump. If a star is to form then the temperature must become high enough to pass the ignition temperature of hydrogen (ie. nuclear burning must start) before the gas pressure stops collapse. If it does not reach this point then the clump is destined to become only a brown dwarf which will eventually cool down again (by radiating its heat away). Assuming the necessary temperatures are reached (on the order of $10^7$K)^1.3 then a star is born. Depending on its mass it will burn for anywhere between around ten thousand years (for a supermassive star) and tens of billions of years (for the smallest red dwarf). The final stages of star formation are for the stellar wind from the new star to blow away the remaining gas and dust leaving behind only the more massive particles which may then go on to form planets.

The timescale for star formation also depends on the mass of the stars formed with the gravitational contraction of the most massive O or B stars taking only around 10,000 years as against up to several 100 million years for the smallest red dwarfs (K type stars) (Pasachoff, p423 [26]).