The life cycle of stars

A star forms in a nebula, a cloud of dust and gas. Gravity pulls the material together into a protostar, which heats up until hydrogen nuclei fuse into helium. The star is then on the main sequence, where the outward pressure from fusion balances the inward pull of gravity. Our Sun will stay on the main sequence for about 10 billion years in total.

When hydrogen runs low, a star like the Sun swells into a red giant, then sheds its outer layers and leaves a white dwarf, which cools into a black dwarf. A star much more massive than the Sun becomes a red supergiant, then explodes as a supernova, leaving a neutron star or, for the largest, a black hole. Fusion in stars makes elements up to iron; heavier elements form in supernovae, which scatter them through space to form new stars and planets.