The LIFE CYCLE of a STAR

Stage 1

They begin as clouds of dust and gas
Denser clumps contract under gravity
As this happens the temperature increases
Eventually the temperature at the centre reaches a few million degrees and hydrogen nuclei fuse together to form helium
This releases large energy (nuclear fusion), and provides enough pressure to stop gravitational collapse
Star has now reached the main sequence
It will stay there in a long stable phase, relatively unchanged

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Main sequence stars become red giants when they run out of fuel
When the hydrogen in the core runs out fusion stops - outward pressure stops
Core contracts, heats up under weight of the star, and then gets hot enough and dense enough for helium to fuse
This releases huge energy, causing the star to expand into RED GIANT - RED SUPERGIANT
They have a high luminosity, low surface temperature - large surface area
This is due to the Stefan - Boltzmann law

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Low mass stars like the Sun will form white dwarfs
Once all helium has fused, low mass stars will collapse under their own weight
The core will not get hot enough for more fusion - just continue to contract
Once Earth size, electrons will exert enough pressure to stop further collapse
As the core contracts the outer layers become more and more unstable
The star pulsates and ejects its outer layers into space as planetry nebula
The star is a hot, dense solid called a WHITE DWARF
They have a low luminosity and high surface temperature

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High mass stars have a shorter life and more exiting death
Large mass - lots of fuel - use it more quickly - less time in main sequence
They will keep fusisng elements untill they are just left with Iron
When it runs out of fuel - contracts to a white dwarf
If its massive enough (1.4 mass of sun), it keeps contracting
The electrons get squashed onto the atomic nuclei - forming neutrons and neutrinos
Core collapses to become a neutron star- which outer layers fall on to
When the outer layers hit the surface they rebound setting up huge shockwaves, ripping the star apart and causing a supernova
The light of which can briefly outshine an entire galaxy

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