# How does a nebula change into a protostar?

A nebula is made of clouds of gas and dust. There are hydrogen gas, of course, and hydrogen atoms. Due to gravity, hydrogen gas tend to clumped together, and the hydrogen atoms will start to spin and pulls up more hydrogen atoms that will cause collision. Collision of the hydrogen atoms causes the hydrogen gas to heat up. When the the temperature reach $15 , 000 , {000}^{\circ} C$, nuclear fusion starts. A protostar is then formed.