How did protostar become that phase?

Answer 1

Protostars form because the cloud of hydrogen and other elements start to compress. Usually this is due to a supernova happening near the nebula which starts it contracting or because two nebula come together.

Answer 2

Usually these are formed in clouds of molecular hydrogen gas. For reasons not fully understood (although most physicists agree that the shockwaves from nearby supernovas can force the hydrogen to clump). Once the clumping begins then it is very easily understood that more and more gas flows toward this source of high density, high mass – a type of runaway effect or domino effect. Once the central pressure reaches a certain point, nuclear fussion begins.