Birth of a star

A star is born in a nebula, a cloud of hot gas, sometimes from a dead star. There are several types of nebulae and these include an emission nebula, dark nebula, planetary nebula and many more. A star forms when the cloud of gas collapses into a dense ball. 

Artists impression of
a T class star

After it has collapsed and more gas has swirled into it, nuclear fusion starts, if the star fails to star it becomes a brown dwarf which are classes L,T,Y. More gas continues to go into the star until it stops growing. The star is born and now creates a gravitational pull to form gas giants and create terrestrial planets.