A protostar is a stage in the evolution of a young star (see star
formation) after it has fragmented from a interstellar gas cloud but
before it has collapsed sufficiently for nuclear fusion reactions to begin. It may last from 100,000 to 10 million years, depending
on the mass of the star. A protostar is surrounded by a dense cocoon of
gas and dust that blocks visible light, but allows through large amounts
of far infrared and microwave radiation.