Evolution of sun like stars - Astrophysics PHYA5 AQA

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Evolution of Sun ­ like Stars
Formation of stars
Stars born in a cloud of dust and gas from old stars that have blown themselves up.
The denser the clumps of cloud contract under force of gravity = protostars
Protostars continue to contract and heat up until it becomes a main sequence star
Hydrogen nuclei fuse into helium = releases energy and creates enough radiation pressure to
stop gravitational collapse.
Core and shell burning sequence
1. Hydrogen burning: main sequence stars ­ pressure produced from hydrogen fusion in their
core balances the gravitational force trying to compress them.
2. Hydrogen in core runs out = nuclear fusion stops = outward pressure stops. Core contracts
and heats up under weight of star which causes outer layers of star to expand and cool =
STAR BECOMES RED GIANT.
3. Shell hydrogen burning: material surrounding core still has hydrogen. High temperature
from the core causes fusion of H He. ( low mass stars stop at this point and use up all their
hydrogen ( fuel) and fade))
4. Core helium burning: Core continues to contract until it gets hot enough for He (already
made in H burning) carbon and oxygen. = releases energy = pushes outer layers of star
outwards further
5. Shell helium burning: He runs out, C-O core contracts again and heats shell around it so He
can fuse in that region
White dwarfs
FOR LOW MASS STARS
C-O core ( shell helium burning stage) won't get hot enough for further fusion = continues to
contract under its own weight
When core shrunk to earth size ( small), electrons exert enough pressure to stop it collapsing
any more ­ ELECTRON DEGENERACY PRESSURE
As core contracts, He becomes more unstable
Star ejects He to outer layers into space as PLANETARY NEBULA, leaving a dense core which
will cool and fade away = WHITE DWARF.

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