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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.