Stars are formed in nebulas, and then begin nuclear fusion. After fusion has progressed significantly, the helium core begins to increase in temperature and fuse to form carbon, at which point the star becomes:

Question

Quizplus · Accepted Answer

After helium fusion begins, the star becomes a red giant, expanding in size and temperature. After the red giant stage, the star can either collapse into a red dwarf, or explode in a supernova.

Stars Are Formed in Nebulas, and Then Begin Nuclear Fusion