Stars spend most of their life cycleson the Main Sequence•Main Sequence stars are in hydrostatic equilibriumbecause nuclear fusion is turning hydrogen intohelium and producing enough outward pressure tobalance gravitational collapse.•90% of all stars are found on the Main Sequence•90% of the whole life of all stars is spent on theMain Sequence•BUT – What happens when the hydrogen runs out?
Stars Leave the Main Sequence•The hydrogen atoms in the core of the star thatfuse together to create helium, start to run outand fusion begins to slow down•The system becomesout of balance•Something has to happen to keep the star fromcollapsing in on itself
Out of balance•Start running out of hydrogen in the core, now theoutward pressure is less than the gravitationalcollapse
Out of balance•What will happen to the core?
When core hydrogen fusion ceases, amain-sequence star becomes a giant•When hydrogen fusion ceases in the core, the starwill collapse inward – thiscauses the layer justoutside the core to become so hot and dense thathydrogen fusion will begin in this outer layer.•The energy produced by hydrogen fusion in thislayer just outside the core causes the rest of thestar to expand into a giant star.•Stellar burp!
Helium fusion begins at the coreof a giant•While the exterior layers expand, the helium corecontinues to contract and eventually becomes hotenough (100 million Kelvin) for helium to beginto fuse into carbon and oxygen–core helium fusion–3 HeC + energyandC + HeO + energy
Main Sequence Stars become Red GiantsHelium fusionHydrogen fusion