All new stars change hydrogen to helium through nuclear fusion. This makes a lot of energy (e.g. light and heat). In a normal star, like our Sun and all other main-sequence stars, this change happens at the very center of the star. Sooner or later, almost all of the hydrogen at the center has changed to helium. This causes the nuclear reaction to stop. The center will start to get smaller due to the star's gravity. This makes the layer just outside the center get hotter. This layer still has hydrogen. This hydrogen will fuse to make helium.