As gravity continues to compress these clumps, they tend toward a spherical shape. Their temperature rises rapidly and the clumps begin to glow from the heat released by the compression of the gases and dust. When they begin to give off visible light, these clumps are now protostars. From the outside these protostars may not be visible because they are still surrounded by an envelope of cooler gases. As the contraction continues, and the temperatures continue to rise, the contraction slows down somewhat because of the internal rise in pressure. 

But fusion pressure rapidly pushes this originally dark envelope outward, leaving behind a bright shiny new star that emerges from this dark opaque cocoon. Depending on its mass, rotation and a host of other factors, the original clump that we are following in this story will give rise to either a single star system, a multiple star system, or a planetary system. 

Usually, the whole clump we mentioned in the nebular stage will form hundreds of stars and more than one clump will be forming stars at the same time in the same region of a nebula. Thus, new stars will be born in open star clusters, whose individual stars will disperse across the galaxy over time as their orbits are affected by the gravitational forces of the galaxy. 

In this early stage of a star's formation, the outward fusion pressure which tends to blow the star up, and the inward pull of gravity which tends to collapse the star, are not balanced yet. This young star is still unstable, occasionally ejecting matter in bursts and varying its light output. But as time goes by, eventually equilibrium between gravity and fusion pressure is reached, and the star enters the stable period of its life, the main sequence stage.