Unstable+stars

When a star runs out of Hydrogen at its core the temperature starts to fall, gravity starts to win the battle. The core becomes squeezed by gravity but this causes the pressure to increase. With an increase in pressure there is an increase in temperature - so finally there will be enough energy to fuse the Helium atoms.

There is a larger amount of energy emitted when Helium is fused, since Helium is a larger atom (remember **E**=mc²) of course you do! :)



The star starts to swell up because there is still Hydrogen fusion in the outer layers - it is here that pressure is dominant (remember gravity decreases the further from the core you go). So in the outer layers pressure is beating gravity.

As increasingly heavier elements are formed at the Hydrogen and Helium burning layers are moved further outwards. It is only at the core that the heavier elements can be formed.



There is still Hydrogen fusion and Helium fusion, but they are now even further away from the core and the star continues to swell!

--> Supernova <-- Stable stars