Chapter 13 An H-R diagram showing the evolutionary track of a one-solar-mass star

Stars form when an interstellar gas cloud's gravity makes it collapse. Collapse compresses the gas and heats it to form a protostar. Eventually the protostar becomes hot enough to fuse hydrogen into helium in its core and becomes a main sequence star. A low mass star like our Sun will burn hydrogen for some 10 billion years. Eventually it uses up its hydrogen and its core shrinks and grows hotter, causing the star's outer layers to expand and cool. This turns the star into a red giant. The core will continue to shrink and grow hotter eventually enabling the star for a few hundred million years to fuse helium atoms into carbon and turning the star from a red giant into a smaller but hotter yellow giant. Eventually the star runs out of helium to fuse and swells again into a red giant, but this time the force of radiation streaming through its outer layers peels them away to form a huge bubble called a planetary nebula. The bubble expands, exposing the star's core, which - with no fuel supply - cools and becomes a white dwarf.