It all starts in a nebula - a massive cloud of dust and gases. Dense regions of the nebula have a greater gravitational pull, and eventually accumulate significant amounts of matter. This matter is compacted under its own gravity, which grows in strength as mass increases. When the force of gravity becomes strong enough, the hydrogen gas collapses in on itself, beginning the process of nuclear fusion. In this stage, the young star is called a protostar. Fusion releases massive ammounts of energy, causing the star to expand against its gravitational pull.