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The Lives of Stars

From studying nearby stars
and stellar clusters



most stars are on the main
sequence
stars become red giants
after leaving the main
sequence
How does this relate to the
internal structure of the
stars and their nuclear
fusion reactions?
Susan Cartwright
Our Evolving Universe
1
Fusion reactions


Generate energy up to iron
But, need to get two positively
charged nuclei close enough to
fuse together

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need fast movement
high temperature
(and high density)
E=mc2
Converting hydrogen-1 to
helium-4 is the easiest and most
efficient fusion reaction

0.7% of initial mass converted
to energy
Susan Cartwright
Our Evolving Universe
2
Stellar structure and fusion

To keep star stable need
pressure to increase
downwards




temperature increases
density increases
fusion most likely in
central core of star
Stars are mainly hydrogen

pprreessssuurree,,
tteem
mppeerraattuurree,,
ddeennssiittyy
iinnccrreeaassiinngg
H
Hee
—>> H
H—
iinn ccoorree
expect main sequence
stars to fuse hydrogen to
helium in core
Susan Cartwright
Our Evolving Universe
3
Hydrogen fusion reactions

Reaction rate
increases as
temperature
increases

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pp chain
more massive stars
have higher fusion
rates
CNO cycle
Reaction can be direct or use carbon-12
as catalyst

this tends to increase abundance
of nitrogen and oxygen
Susan Cartwright
Our Evolving Universe
4
Getting the heat out
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Energy generated in stellar core has to be transported to
surface
Two options:

radiation


convection


absorption and re-emission
of photons
hot gas rising towards surface,
cool gas falling
Giant stars have convective outer layers

transports out the heavy elements
produced by fusion
Susan Cartwright
Our Evolving Universe
5
What happens when the core
hydrogen runs out?
star expands and cools,
becoming luminous
red giant (1000x Sun)
1.76 Gyr
core shrinks under gravity
until hydrogen outside
starts to fuse
1.65 Gyr
1.69 Gyr
on main sequence
10x Sun
Susan Cartwright
star now has helium core 1.61 Gyr
(not hot enough to fuse)
Our Evolving Universe
6
Stages of hydrogen fusion

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

Main sequence stars fuse
hydrogen to helium in core
Red giants (and subgiants)
fuse hydrogen to helium in
shell outside helium core
Stars have nearly constant
luminosity on main sequence,
but red giants get brighter
as they age
Red giant stage lasts only
10% as long as main sequence
Susan Cartwright
established
fusion in
shell
core hydrogen fusion
Our Evolving Universe
starting to
fuse outside core
7
Helium fusion

Neither beryllium-8 nor
boron-8 is stable



need to combine three
helium nuclei to get stable
carbon-12
beryllium-8 serves as
intermediate stage
need high temperature and
density (else 8Be decays
before it gets converted
to 12C)
Susan Cartwright
Our Evolving Universe
8
Stages of helium fusion
carbon core with helium
fusion outside: star
becomes a giant again
helium fusion in core:
star is smaller and
hotter, but less bright
Susan Cartwright
1.82 Gyr
Our Evolving Universe
1.86 Gyr
core helium
starts to
fuse
1.76 Gyr
9
Helium fusion
on the HR diagram

Helium fusion is much less
efficient than hydrogen fusion
(0.07% instead of 0.7%)

helium fusion stage lasts for a
much shorter time
helium core
fusing stars:
the horizontal branch
of a globular
cluster
Susan Cartwright
Our Evolving Universe
10
Side effects of helium fusion

Adding more helium nuclei
to carbon can produce the
alpha-process elements

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Why does helium fusion
make mostly carbon?

oxygen-16, neon-20, etc.
Adding helium to carbon-13
or neon-22 produces free
neutrons

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which can easily combine
with nuclei (no charge) to
produce different
elements
Susan Cartwright


Our Evolving Universe
because carbon nuclei have
an energy level at exactly
the right place
otherwise carbon would be
a rare element
and we would not exist!
Fred Hoyle, 1953
11
Stellar evolution
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Note step is in log (age):
each frame is 60% older
than the one before
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massive stars evolve very
quickly
post-main-sequence life of
star is always comparatively
short
massive stars change colour
a great deal, but don’t
change brightness much
less massive stars become
much brighter as red giants
Susan Cartwright
Our Evolving Universe
12
After helium fusion

Fusion of heavier elements
gets more difficult
higher mass means lower
speed at given temperature
higher charge means more
electrostatic repulsion
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Stars like the Sun never get
beyond helium fusion
More massive stars (>8 MS)
can fuse elements up to iron
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What happens to Sun-like
stars when the helium is
used up?
What happens to massive
stars when they reach iron?
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fusion beyond iron requires
energy
How are the heavy elements
formed in stellar cores
dispersed into space?
…next lecture!
Susan Cartwright
Our Evolving Universe
13