The Evolution of Stars Stars form dense clouds of gas and dust. Gravity condenses part of the cloud into a warm, red protostar. As the core of the prostar grows hotter, it glows a bright red. The prostar spins and is flattened into a disk. The infant star remains at the core, and planets may form from rings in the disk.
Contraction heats the core until nuclear fusion of hydrogen into heluim begins.
The new star becomes a main sequence star and shines for billions of years. How long it stays a main sequence star depends on its mass. As the hydrogen becomes spent, the star's core contracts more, but the outer layers swell to an enormous size, forming a red giant. These layers eventually blow off into an expanding cloud of gas forming a planetary nebulae. These gas clouds cast off by dying stars glow with fluorescent light for 100,000 years before they fade.