the lives of stars
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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?. Fusion reactions. - PowerPoint PPT PresentationTRANSCRIPT
Our Evolving Universe 1Susan Cartwright
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?
HR Diagram of nearby stars
-5
0
5
10
15
20-0.5 0 0.5 1 1.5 2 2.5
B - V
abso
lute
vis
ual m
agni
tude
Our Evolving Universe 2Susan Cartwright
Fusion reactions
Generate energy up to iron But, need to get two
positively charged nuclei close enough to fuse together
need fast movement high temperature
(and high density) Converting hydrogen-1 to
helium-4 is the easiest and most efficient fusion reaction
0.7% of initial mass converted to energy
Nuclear stability
0.998
0.999
1
1.001
1.002
1.003
1.004
1.005
1.006
1.007
1.008
1.009
0 50 100 150 200Number of nucleons
Mas
s pe
r nu
cleo
n
helium 4
hydrogen 1
iron 56
E=mc2
Our Evolving Universe 3Susan Cartwright
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 expect main sequence
stars to fuse hydrogen to helium in core
HHH ———>>> HHHeeeiiinnn cccooorrreee
ppprrreeessssssuuurrreee,,,ttt eeemmmpppeeerrraaatttuuurrreee,,,dddeeennnsssiiittt yyyiiinnncccrrreeeaaasssiiinnnggg
Our Evolving Universe 4Susan Cartwright
Hydrogen fusion reactions
Reaction rateincreases astemperatureincreases
more massive starshave higher fusionrates
Reaction can be direct or use carbon-12as catalyst
this tends to increase abundanceof nitrogen and oxygen
pp chain
CNO cycle
Our Evolving Universe 5Susan Cartwright
Getting the heat out
Energy generated in stellar core has to be transported to surface
Two options: radiation
absorption and re-emissionof photons
convection hot gas rising towards surface,
cool gas falling Giant stars have convective outer layers
transports out the heavy elementsproduced by fusion
Our Evolving Universe 6Susan Cartwright
What happens when the core hydrogen runs out?
0
0.5
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1.5
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2.5
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3.5
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3.53.553.63.653.73.753.83.853.93.95
log T
log
L
Star of 1.7 solar masses
star now has helium core (not hot enough to fuse)
on main sequence10x Sun
core shrinks under gravityuntil hydrogen outside starts to fuse
star expands and cools,becoming luminousred giant (1000x Sun)
1.61 Gyr
1.65 Gyr
1.69 Gyr
1.76 Gyr
Our Evolving Universe 7Susan Cartwright
Stages of hydrogen fusion
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
M67
6
7
8
9
10
11
12
13
14
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16-0.5 0 0.5 1 1.5 2
B-V
V
core hydro-gen fusion
starting to fuse out-side core
establishedfusion in
shell
Our Evolving Universe 8Susan Cartwright
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)
Our Evolving Universe 9Susan Cartwright
Stages of helium fusion
0
0.5
1
1.5
2
2.5
3
3.5
4
3.53.553.63.653.73.753.83.853.93.95
log T
log
L
Star of 1.7 solar masses
core heliumstarts to
fuse
helium fusion in core:star is smaller and
hotter, but less bright
carbon core with heliumfusion outside: star
becomes a giant again
1.86 Gyr
1.76 Gyr
1.82 Gyr
Our Evolving Universe 10Susan Cartwright
M3
12
13
14
15
16
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18
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20
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22-0.3 0 0.3 0.6 0.9 1.2
B-V
V
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
Nuclear stability
0.998
0.999
1
1.001
1.002
1.003
1.004
1.005
1.006
1.007
1.008
1.009
0 50 100 150 200Number of nucleons
Mas
s pe
r nu
cleo
n
helium 4
hydrogen 1
iron 56
helium corefusing stars:
the hori-zontal branchof a globular
cluster
Our Evolving Universe 11Susan Cartwright
Side effects of helium fusion
Adding more helium nuclei to carbon can produce the alpha-process elements
oxygen-16, neon-20, etc. Adding helium to carbon-13 or neon-22 produces free
neutrons which can easily combine with nuclei (no charge) to
produce different elements
Why does helium fusion make mostly carbon? 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
Our Evolving Universe 12Susan Cartwright
Stellar evolution
Note step is in log (age): each frame is 60% older than the one before
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
Our Evolving Universe 13Susan Cartwright
After helium fusion
Fusion of heavier elements gets more difficult
higher mass means lower speed at given temperature
higher charge means more electrostatic repulsion
Stars like the Sun never get beyond helium fusion
More massive stars (>8 MS) can fuse elements up to iron
What happens to Sun-like stars when the helium is used up?
What happens to massive stars when they reach iron?
fusion beyond iron requires energy
How are the heavy elements formed in stellar cores dispersed into space?
…next lecture!