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Sternentwicklung
Stellar evolution
Vorlesung Spätstadien der
Sternentwicklung, WS 07/08
Lebzelter & Hron
Early concepts
Lord Kelvin:
source of solar energy is gravitational contraction.
Age of the sun is 100 million years
Charles Darwin:
age of the earth is several billion years.
Ernest Rutherford:
Radium possible long time energy source
O. Gingerich, 1999, Ap&SS 267, 3
Early concepts
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Star form out of meteoritic particles, become first red giants, contract
(hot stars), and then cool down (red dwarfs).
Lockyer (1890), Russell (1925)
Milestones
• Understanding of the stellar structure
(Eddington)
• Understanding of the star‘s composition
(Payne, Unsöld)
• Understanding of the energy source
(Atkinson, Bethe, von Weizsäcker)
What is a star?
Energy Production
Nuclear Timescale
E nuklear
tnuklear 
LSonne
E nuklear  0.1 0.007  M Sonnec 2

10% of the sun involved

E = m c2
0.7% of the mass of a hydrogen
core will be converted into energy
tnuclear ~ 1010 years
core hydrogen burning
Effect of hydrogen burning
• 4 H transformed into 1 He
• mean molecular weight increases
• according to the ideal gas law density and
T have to increase:
kT
Pgas 
mH
• core contracts  energy production
increases and opacity decreases  L, R, T
increase
Iben 1967
Core after H burning
• no H  no energy production
• energy production in shell, core becomes
isothermic
• H burning shell  core increases in mass
• maximum core mass ~ 10% stellar mass
 core collapse
• low mass stars:
core degenerates first
Core and Envelope
• Lbottom = Lout  star is in TE
• Lbottom increases  Lout has to increase
• R increases  more surface  more L
can be emitted  again TE
• R increases  T decreases  at some
point opacity increases  NO TE  star
becomes red giant (Hertzsprung-gap!)
• Runaway stops at Hayashi-track
Hayashi line
TE
runaway
phase
Energy trapped
no TE
TE
convection
TE
Iben 1967
First Dredge Up
• Convective zone reaches layers with
processed material
• Abundance changes on the surface:
12C decreases
Li decreases
14N increases
3He increases
O ~ constant
• 12C/13C drops from 90 (solar) to ~20
Evolution on the RGB
• Luminosity provided almost exclusively by
thin H burning shell (0.001 – 0.0001 Msolar)
• burning rate of H shell determined by size
and mass of core
•  He core mass – luminosity relation
Helium core flash
• core T increases until He ignition
temperature (108 K) – approx. 0.5 Msolar
• core material degenerated  gas pressure
not sensitive to T  no cooling by
extension  thermonuclear runaway
• Duration approx. 1 Mio years
• Most of the E does not reach the surface
• not for stars above 2.25 Msolar
Helium Burning
Helium Burning
• 4He + 4He  8Be
8Be + 4He  12C
• Energy production  T40
• Energy release per nucleus one order of
magnitude less than for H burning
• 12C +   16O + 
16O +   20Ne + 
He exhausted 
2nd ascent on the giant
branch (Asymptotic Giant
Branch, AGB)
Our Sun
Ejection of outer shell
012Milliarden
Jahre
4.5
Billion years
years
12.4
10
Billion
Helium
ignition
Birth
Hydrogen exhausted
Today
5600 4500
4000
3200
Aus Sackmann et al.
1993
CMD of Stellar Clusters
log L/Lsun
Isochrones
5
5
5
4
4
4
3
3
3
2
2
2
1
1
1
0
0
0
-1
-1
-1
-2
-2
8 Gyr
Z=0.0004
-3
4.4
4.2
4.0
3.8
log Teff
Bertelli et al. 2000
3.6
-3
4.4
-2
9 Gyr
Z=0.0004
4.2
4.0
3.8
log Teff
3.6
-3
4.4
10 Gyr
Z=0.0004
4.2
4.0
3.8
log Teff
3.6
log L/Lsun
Isochrones
4
4
4
3
3
3
2
2
2
1
1
1
0
0
0
-1
-1
-1
-2
-2
10 Gyr
Z=0.001
-3
3.9
3.8
3.7
log Teff
3.6
-3
3.9
-2
10 Gyr
Z=0.0004
3.8
3.7
log Teff
3.6
-3
3.9
10 Gyr
Z=0.0001
3.8
3.7
log Teff
3.6
Literature
• Salaris & Cassisi: Evolution of Stars and
Stellar Populations
• Kippenhahn & Weigert: Stellar Structure
and Evolution
• Renzini et al. 1992, ApJ 400, 280
Miras Innenleben
helium burning
Aus: James Kaler, Sterne
Beiträge zum ISM
100
% 10
1
TP-AGB
SN
RGB
WR
R,YSG
E-AGB
MS
Sedlmayr 1994
Thermische Pulse
PDCZ...Pulse driven convection zone
Thermische Pulse
continuous line...surface luminosity
dotted line...He-burning luminosity
dashed line...H-burning luminosity
Wood & Zarro 1981
Vassiliadis & Wood 1993
Wood & Zarro 1981
shell hydrogen burning