What critical event must occur in oder for a protostar to become a star?

Question

What critical event must occur in oder for a protostar to become a star?

Answer 1

The temperature and pressure must be adequate to sustain nuclear fusion.

Answer 2

Ignition.

Answer 3

From the web: A protostar is a large object that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a solar-mass star it lasts about 100,000 years. It starts with a core of increased density in a molecular cloud and ends with the formation of a T Tauri star, which then develops into a main sequence star. This is heralded by the T Tauri wind, a type of super solar wind that marks the change from the star accreting mass into radiating energy.

Observations reveal that giant molecular clouds are approximately in a state of virial equilibrium—on the whole, the gravitational binding energy of the cloud is balanced by the thermal pressure of the cloud's constituent molecules and dust particles. Although thermal pressure is likely the dominant effect in counteracting gravitational collapse of protostellar cores, magnetic pressure, turbulence and rotation can also play a role (Larson, 2003). Any disturbance to the cloud may upset its state of equilibrium. Examples of disturbances are shock waves from supernovae; spiral density waves within galaxies and the close approach or collision of another cloud. If the disturbance is sufficiently large, it may lead to gravitational instability and subsequent collapse of a particular region of the cloud