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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
5. Shell helium burning: He runs out, C-O core contracts again and heats shell around it so He
can fuse in that region
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.