Lecture 20: Understanding stars part 03 HR diagram Main sequence: stars lives while they are fusing hydrogen and helium. But what happens after they run out of fuels? They move towards red giants. Hydrostatic equilibrium: need high temperature to support the pressure. We need constant energy to keep the star hot, through fusion. Use up the hydrogen: no fusion, no pressure to balance gravity, start shrinking. The center is a core thats helium ash. It shrinks, and core is smaller. As the core gets smaller, it heats up again due to compression. The material around the core is also heating up, so that region is now hot enough to fuse hydrogen into helium: called shell burning. As it gets hotter, the shell gets hotter and more luminous. Its fusing so rapidly that its producing more pressure than needed to balance the stars gravity, so it becomes a giant. The surface gets cooler from such amount of shrinking, so it turns red. Why does the shell still have hydrogen? Only the central core is hot enough to undergo fusion when its still in the main sequence. The inner layer dont mix much with the core! HR diagram: move to the right. More mass, more temperature, and more luminous. P.S.: in expansion, the surface of the Sun feels less gravity from the center, thus creating the wind, driving materials to escape. Loss of some mass at this point. Helium fusion: to settle down and use helium as fuel; very hard. Beryllium8 isnt stable, and it falls apart again. In it brief span, we need another helium to turn it into a carbon12, at basically the same time. We need 100 Million K for this to happen. It happens after a tip of the red giant branch to fuse helium into carbon.