For most of their lives, stars fuse hydrogen to helium at just the rate needed to push back against the gravity that crushes them. Gravity crams the hydrogen into the star’s core, heating it mercilessly. The energy released when the protons fuse into helium pushes back against gravity, holding the outer layers up against gravity. For stars the size of our Sun, this balance holds for billions of years, but eventually the hydrogen is used up. Then gravity wins, and the star begins to fall into its core, getting much hotter. Eventually, the heat and density are enough that the helium can fuse. This releases much more heat, puffing the star out into an enormous Red Giant.
Red giant stars like Antares or Arcturus are bigger than the orbit of Saturn. Helium is fused to carbon in their cores, then carbon into oxygen, then oxygen into iron. The outer layers of red giant stars are incandescent plasma more tenuous than a laboratory vacuum, barely held by the curved space of the star’s mass. These outer layers fluff off into the void, releasing lovely envelopes of glowing gas like the Ring Nebula. Small stars like our Sun end their lives when the core exhausts its fuel and loses its hold on the expanding envelope. The hot remnant core is laid bare as a white dwarf, cooling slowly almost till eternity.
Bigger stars will keep fusing heavier and heavier elements until something terrible happens — a supernova. These colossal explosions can be as bright as the whole galaxy for a week or so, spewing carbon, nitrogen, oxygen, silicon, and iron into interstellar space.
