Stellar Evolution

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Stellar Evolution
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What is the expected main-sequence life time of
the sun?

• 10 million years
• 100 million years
• 1 billion years
• 10 billion years
• 100 billion years

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Evolution on the Main Sequence (I)
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Main-Sequence stars live by fusing H to He.
MS evolution
Zero-Age Main Sequence (ZAMS)
Finite supply of H gt finite life time.
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Evolution on the Main Sequence (II)
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Evolution off the Main Sequence Expansion into a
Red Giant
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H in the core completely converted into He
H burning (i.e. fusion of H into He) continues
in a shell around the core.
Expansion and cooling of the outer layers of the
star ? Red Giant
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Question
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B
A
Which way will a star at point X move in the HR
diagram when it swells up to a red giant, so that
its luminosity increases slightly, and its color
changes to red?
C
X
D
E
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Which way will the star move in the HR diagram?

• A
• B
• C
• D
• E

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Expansion onto the Giant Branch
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Expansion and surface cooling during the phase of
an inactive He core and a H- burning shell
Sun will expand beyond Earths orbit!
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Red Giant Evolution
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He-core gets denser and hotter until the next
stage of nuclear burning can begin in the core
4 H ? He
He
He fusion 3 4He ? 12C Triple-Alpha
Process Fusion of Helium into Carbon
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Red Giant Evolution (5 solar-mass star)
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C, O
Inactive He
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The cycle of fusion can, in principle continue
until which element is reached?

• Helium
• Carbon
• Oxygen
• Iron
• Plutonium

H ? He
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He ? C, O
C ? Ne, Mg, O
Ne ? O, Mg

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The Life Clock of a Massive Star (gt 8 Msun)
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Lets compress a massive stars life into one day
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H ? He
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Life on the Main Sequence Expansion to Red
Giant 22 h, 24 min. H burning
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H ? He
He ? C, O
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He burning (Red Giant Phase) 1 h, 35 min, 53 s
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He ? C, O
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H ? He
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C ? Ne, Na, Mg, O
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C burning 6.99 s
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C ? Ne, Na, Mg, O
H ? He
Ne ? O, Mg
He ? C, O
Ne burning 6 ms
235959.996
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C ? Ne, Na, Mg, O
H ? He
Ne ? O, Mg
He ? C, O
O ? Si, S, P
O burning 3.97 ms
235959.99997
C ? Ne, Na, Mg, O
H ? He
Ne ? O, Mg
He ? C, O
O ? Si, S, P
Si ? Fe, Co, Ni
The final 0.03 msec!!
Si burning 0.03 ms
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Summary of Post-Main-Sequence Evolution of Stars
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Fusion proceeds to formation of Fe core.
Evolution of 4 - 8 Msun stars is still uncertain.
Fusion stops at formation of C,O core.
M gt 8 Msun
Red dwarfs He burning never ignites
M lt 4 Msun
M lt 0.4 Msun
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Evidence for Stellar Evolution Star Clusters
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Stars in a star cluster all have approximately
the same age!
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Take all stars of a 5-billion-year-old cluster,
and put them onto a Hertzsprung-Russell diagram.
Do you expect to see a complete Main Sequence?

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• Yes
• No, stars near the upper end will be missing.
• No, stars near the lower end will be missing.

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High-mass stars evolve off the main sequence (to
become red giants) earlier than low-mass stars.
gt For a given age, low-mass stars are still on
the MS, while high-mass stars are already red
giants!
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ExampleHR diagram of the star cluster M 55
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High-mass stars evolved onto the giant branch
Turn-off point
Low-mass stars still on the main sequence
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What can we infer from the location of the
turn-off point?

• The average mass of stars in the cluster.
• The distance of the cluster.
• The age of the cluster.
• The size of the cluster.
• All of the above.

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The lower on the MS the turn-off point, the older
the cluster.
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Evidence for Stellar Evolution (II) Variable
Stars
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Some stars show periodic brightness variations.
Most important example d Cephei
Light curve of d Cephei
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Cepheid VariablesThe Period-Luminosity Relation
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The variability period of a Cepheid variable is
correlated with its luminosity.
The more luminous it is, the more slowly it
pulsates.
gt Measuring a Cepheids period, we can determine
its absolute magnitude!
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What can we infer from a Cepheids absolute
magnitude? (Of course, we can also measure its
apparent magnitude.)

• Its distance.
• Its age.
• Its mass.
• Its temperature.
• Its radius.

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Comparing absolute and apparent magnitudes, we
can measure a stars distance (using the 1/d2
law)!
The Cepheid distance measurements were the first
distance determinations that worked out to
distances beyond our Milky Way!
Cepheids are up to 40,000 times more luminous
than our sun gt can be identified in other
galaxies.