A medium-sized star is one that ranges from about one to three solar masses and follows the main sequence of a star's lifetime. Medium-sized stars, such as the sun, can live for billions of years, while their larger counterparts burn out more quickly.
A medium star spends most of its time in the main sequence, or the method by which it fuses hydrogen into helium to create energy. The star continues to fuse all its hydrogen into helium and is considered to be in the "adulthood" phase of its life cycle during this time. The sun is expected to last for about five billion more years before using up all its hydrogen and turning into a red giant. This happens when there is no more energy to resist the pull of gravity.
The star then collapses and begins fusing helium, which does not produce as much energy as hydrogen fusion. The star expands to hundreds or thousands of times its original size, then collapses into a white dwarf when its helium is fused into carbon and iron. This fusion process requires more energy than it yields. White dwarf stars glow with residual heat until they radiate all their heat away into space.