Gravity pulls star together Contracts and is heated up Ignites Radiation pressure balances gravity: 4 H What happens after all the hydrogen is gone? Star collapses: inside heats up very hot Helium “ignites”: heavier elements build up Excess heat blows up the star large – becomes a red giant (cold outside) Why very hot inside? Needs extra high temperature to run against increased Coulomb barrier. Why small core? Needs to be small to increase gravity in order to maintain extra high temperature. Why bright? Needs extra power to hold up against increased gravity. Why large? Extra radiation pressure blows out atmosphere. Why cold atmosphere?