It is all matter of what kind of stars are we talking about, for simplicity let's say we are talking about normal stars such as our sun.
If there is a molecular cloud that has a mass that is slightly larger than our sun then it is possible that the gravity will eventually pull together cloud into a sphere that would have enough mass to start nuclear fusion which is important to become a star.
Mass of such cloud would be 1.98x10^30Kg almost equal to the sun's mass.
All of this implies that stars are formed when there is enough mass to let gravity pull it all together into a sphere that has enough gravitational pull to start nuclear fusion inside the core.
