The Life and Death of Stars.pdf - Astronomy 103 The Lives...

Astronomy 103
The Lives and Deaths of Stars, and the Origin of
the Elements

A star like our sun might live for 10 billion years. Some stars
lives for 4 million years. So how do we know how stars live
and die if humans have only been studying stars for the last
2000 years?

For stars like the sun, their evolu+on can be broken into six
stages (in Roman numerals)
I. Collapse of a gas cloud to form a star
II. Main Sequence
III. Red Giant
IV. Helium Flash
V. Second Red Giant (Asympto+c Giant Branch [AGB])
VI. Planetary Nebula and forma+on of White Dwarf
It is helpful to keep in mind that stars are a ba;le between
gravity and nuclear energy. And gravity always wins in the
end.

II. Main sequence star
The main sequence is
boring, where H fuses in
the core to make He.
But where a star sits on
the main sequence
depends on its mass.

The energy of a main sequence star is
produced by fusion of H to He in the star’s
core
II. Main sequence

Hydrosta±c balance
Recall that stars are a balance
between pressure pushing stu'
out and gravity pulling stu' in.
Because, star are constantly
losing energy, they would
contract if it were not for a heat
source (nuclear fusion) in the
center.

At some point, the stars will
run out of fuel. But it will
s+ll shine and so gravity
takes over: the core again
begins to contract.
As it contracts, the core
heats up (remember the
nega+ve heat capacity – if
you cool it, it gets ho;er).

At some point the
star gets hot enough
to burn hydrogen in
the outer layer
around the He core.
This is called
Hydrogen shell
burning
This next stage is the
Red Giant stage.
II. Red Giant

II. Red Giant
Because the core is denser and ho;er than before, the H
fusion is much faster in the shell and the energy output is
much greater than when the star was on the main
sequence.

The huge amount of energy pouring out from the central
region of the star in?ates the outer star like a balloon:

Why is a red giant red?

Why is a red giant red?