The stage in the evolution of a star between fragmentation and the zero age main sequence state, where nuclear reactions begin.

Stars are believed to be born in groups from the collapse of large, cold clouds of interstellar material composed primarily of hydrogen gas.

A protostar becomes a proper star when it begins fusing hydrogen into helium in its core. A star of one solar mass fuses its hydrogen for about ten billion years.

According to this analysis, the sun, which is about 4.6 billion years old, is a middle-aged star.