Stellar Evolution

The whole process takes about 10 millions years. If a protostar forms with a mass less than 0.08 solar masses, its internal temperature never reaches a value high enough for thermonuclear fusion to begin. This failed star is called a brown dwarf, halfway between a planet (like Jupiter) and a star.

Brown dwarfs still emit energy, mostly in the IR, due to the potential energy of collapse converted into kinetic energy. Brown dwarfs eventual fade and cool to become black dwarfs.

The helium ash builds up, energy generation stops in the core. The fusion process moves outward into a shell surrounding the hot helium core. Helium can also undergo fusion but, since it is a larger atom, it requires over a 100 million degrees of temperature. For small stars, this temperature is never reached and the helium core remains inert.

Carbon build in the core. Thin layer of burning helium surrounding. IF the mass of the star is less than four solar masses- carbon fusion will not take place. Core collapses under gravity. Outer layers are ejected Result is Planetary Nebula.

Core contains electrons and carbon nuclei. IF the core is less than 1.4 solar masses- White dwarf Energy radiates until the star “cools” Black Dwarf.

Alternate story If the White dwarf has a companion: Matter is swept into the WD via intense gravitational field. Explodes on the surface of the WD. Eventually WD becomes unstable due to additional mass.

White Dwarf could eventually explode. Type I Supernova