The Death of Stars.ppt - The Evolution and Death of Stars...

The Evolution and Death of Stars

Stars spend most of their life cycles
on the Main Sequence
•
Main Sequence stars are in hydrostatic equilibrium
because nuclear fusion is turning hydrogen into
helium and producing enough outward pressure to
balance gravitational collapse.
•
90% of all stars are found on the Main Sequence
•
90% of the whole life of all stars is spent on the
Main Sequence
•
BUT – What happens when the hydrogen runs out?

Stars Leave the Main Sequence
•
The hydrogen atoms in the core of the star that
fuse together to create helium, start to run out
and fusion begins to slow down
•
The system becomes out of balance
•
Something has to happen to keep the star from
collapsing in on itself

Out of balance
•
Start running out of hydrogen in the core, now the
outward pressure is less than the gravitational
collapse

Out of balance
•
What will happen to the core?

When core hydrogen fusion ceases, a
main-sequence star becomes a giant
•
When hydrogen fusion ceases in the core, the star
will collapse inward – this causes the layer just
outside the core to become so hot and dense that
hydrogen fusion will begin in this outer layer.
•
The energy produced by hydrogen fusion in this
layer just outside the core causes the rest of the
star to expand into a giant star.
•
Stellar burp!

Helium fusion begins at the core
of a giant
•
While the exterior layers expand, the helium core
continues to contract and eventually becomes hot
enough (100 million Kelvin) for helium to begin
to fuse into carbon and oxygen
–
core helium fusion
–
3 He  C + energy
and
C + He  O + energy

Main Sequence Stars become Red Giants
Helium fusion
Hydrogen fusion

As stars evolve,
stars move from
being main
sequence stars to
Red Giants where
they
increase in
luminosity and
brightness and
decrease in
temperature

Interstellar
Cloud (gas
and dust)
Main
Sequence
Star
Red
Giant
The Life of a Star

Main Sequence Stars become Red Giants
Helium fusion
Hydrogen fusion