Stellar structure and evolution
Pierre Hily-Blant
IPAG
[email protected], OSUG-D/306
2016-17
P. Hily-Blant (Master PFN)
Stellar structure and evolution
2016-17
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5 Stellar evolution overview
Lecture 5
Stellar evolution overview
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Stellar structure and evolution
2016-17
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5 Stellar evolution overview
1. Stellar populations
5– Stellar evolution overview
Stellar populations
H-R diagram
Main evolution phases
P. Hily-Blant (Master PFN)
Stellar structure and evolution
2016-17
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5 Stellar evolution overview
1. Stellar populations
Stellar populations
Population II
• Oldest stars in our Galaxy, often in clusters;
• Galaxy’s spherical halo
• Also called late type stars
• Galaxy formation: mostly H, small amount of He, traces of metals
• Extremely metal-poor: 10−4 < Z /Z < 10−3
• Ultra metal-poor: 10−5 < Z /Z < 10−4
• Hyper metal-poor: 10−6 < Z /Z < 10−5
• Pop II clusters: globular clusters
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5 Stellar evolution overview
1. Stellar populations
Stellar populations
Population I
• Formed recently, in the Galactic disk
• Early type stars
• The youngest stars are relatively rich in heavy elements
• Pop I clusters: galactic clusters
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5 Stellar evolution overview
1. Stellar populations
Stellar populations
Population III
• Postulated; no observational evidence so far; would be...
• ... the first stars in the Universe, at the end of the cosmic ’dark ages’
• ... formed prior to the first galaxies
• ... formed from zero metallicity (Z = 0) gas within dark-matter
minihalos (T ∼ 1000 K and M ∼ 106 M )
• z ≈ 20 − 30, age of the Universe ∼ 108 yr
• Enriched the pure H/He early Universe
• Inject energy from the Pop III strs and first BH
• III.1 (first luminous objects) and III.2 (formed from metal-free gas
photoionized by III.1 before gravitational collapse)
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Stellar structure and evolution
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5 Stellar evolution overview
2. H-R diagram
5– Stellar evolution overview
Stellar populations
H-R diagram
Main evolution phases
P. Hily-Blant (Master PFN)
Stellar structure and evolution
2016-17
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5 Stellar evolution overview
2. H-R diagram
Hertzsprung-Russell diagram
• 22000 stars from Hipparcos
catalog
• 1000 stars from Gliese (d < 25
pc)
• L = 4πR 2 σT 4
• L ∝ M α, α ≈ 3
• R ∝ M ξ , ξ = 0.57 − 0.80
• Lifetime on MS:
tMS ∝ M/L ∼ M −2
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5 Stellar evolution overview
2. H-R diagram
Hertzsprung-Russell diagram
• ESA/GAIA: in progress
• New distances
• New fluxes, magnitudes
• New sizes
• etc...
YOU !
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5 Stellar evolution overview
2. H-R diagram
H-R diagram: main features
(1) main sequence (MS)
• Spica A to L726-8 B: most of the stars
• (gas+radiation) pressure
counteracting gravity
• core H-burning stars: H to He
conversion
(2) white dwarfs (WD)
• Sirus B to L870-2
• small radius
• support: degenerate electron gas
pressure
• luminosity: loss of thermal energy by
non-degenerate nuclei
Iben (1991)
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Stellar structure and evolution
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5 Stellar evolution overview
2. H-R diagram
Hertzsprung-Russell diagram
(3) core He-burning band: Rigel to Capella
• He burning in the core
• luminosity: H-burning in a shell
(4) red giants or red supergiants
•
•
•
•
•
Betelgeuse down to Aldebaran
down to MS (subgiants), RS CVn
name: colors and size
luminosity: H-burning in a shell
support: He-burning core, or compact
core
Iben (1991)
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5 Stellar evolution overview
2. H-R diagram
Giants
• Giants and supergiants: H-burning in a
shell, He-burning in the core
• Giants of intermediate luminosity, or
pulsing AGB stars (Mira): H- and
He-burning in shells above WD-like core
of C and O
• Weak Giants (Pollux, Aldebaran): shell
H-burning on a compact hot core of
size ∼ WD;
Iben (1991)
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2016-17
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5 Stellar evolution overview
3. Main evolution phases
5– Stellar evolution overview
Stellar populations
H-R diagram
Main evolution phases
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5 Stellar evolution overview
3. Main evolution phases
Clusters
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5 Stellar evolution overview
3. Main evolution phases
Clusters: population evolution
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5 Stellar evolution overview
3. Main evolution phases
Clusters
• h + χ Persei: turn-off point +
bright red stars; stars turning off
are less than 2 Myr
• M67: ∼ 3 − 4 Gyr; giant branch
• Initial mass function (IMF)
• Common feature: at turn-off
point, stars are brighter and
4 ; R
redder; but L = 4πR 2 σTeff
increases (×10 − 100)
Different clusters
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5 Stellar evolution overview
3. Main evolution phases
Globular clusters
M3 cluster, 10 Gyr
• MS: main sequence
• TO: turn-off point
• SGB: sub-giant branch
• RGB: red-giant branch
• AGB: asymptotic giant branch
• HB: horizontal branch
• P-AGB: post-AGB (e.g.
planetary nebula)
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Stellar structure and evolution
2016-17
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5 Stellar evolution overview
3. Main evolution phases
Globular clusters
M3 cluster, 10 Gyr
• MS: main sequence
• TO: turn-off point
• SGB: sub-giant branch
• RGB: red-giant branch
• AGB: asymptotic giant branch
• HB: horizontal branch
• P-AGB: post-AGB (e.g.
planetary nebula)
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Stellar structure and evolution
2016-17
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5 Stellar evolution overview
3. Main evolution phases
Overview
• Formation of a gravitationally bound core
• Which becomes unstable and undergoes gravitational collapse
• Pre-main sequence star: protostar approaching the MS
• ZAMS: H-burning starts in the core
• MS: ∼90% lifetime; slow evolution, nuclear burning, static
• Evolution: burning fuel until exhausted, gravitational collapse,
density/temperature adjustment (EOS, contraction/expansion),
nuclear burning in shells, ignition of a new fuel in the core (if any !),
until
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Stellar structure and evolution
2016-17
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5 Stellar evolution overview
3. Main evolution phases
Pre-main sequence stars
• Contraction
• Mainly isothermal
• Hydrostatic equilibrium
• Timescale: Kelvin-Helmoltz
• tKH ∼ 107 M 2 /RL yr
• tKH decreases with M
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Stellar structure and evolution
2016-17
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5 Stellar evolution overview
3. Main evolution phases
Main sequence stars
• H-burning phase
• ∼ 90% of star’s lifetime
• Nuclear timescale ∼ 1010 M/L yr ∼ 1016 dynamical timescales:
quasi-static evolution
• Energy transport
• M < 1.3 M : lower MS, energy from pp chain, core is radiative
• M > 3 M : upper MS, energy from CNO cycle, core is convective
• intermediate mass: both convective core + radiative surrounding
• Convective zone; convection: element mixing within the core
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Stellar structure and evolution
2016-17
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5 Stellar evolution overview
3. Main evolution phases
Evolution off the MS
a) Nuclear burning increases µ (mean mol. wei.): P decreases in the
energy production zone, core contraction, T and P increase, to
restore hydrostatic equilibrium
b) Increasing T , increase of energy generation: luminosity increases;
radius increases
c) Onion-skin structure: rising T enables ignition of higher-mass fuel in
the core; surrounding it are inert ashes; surrounding is a shell where
previous burning continues; shell of unignited material; ... ; until the
H-envelope.
• nuclei of increasing Z are burned
• inhomogeneous structure (onion-skin model)
• each new burning phase shortens
• comparable timescales, coupling become important; no longer
quasistatic
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5 Stellar evolution overview
3. Main evolution phases
Onion-skin model
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5 Stellar evolution overview
3. Main evolution phases
Evolution: overview
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Stellar structure and evolution
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5 Stellar evolution overview
3. Main evolution phases
Stellar embryos
Eagle nebula with the HST
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5 Stellar evolution overview
3. Main evolution phases
Young stars
NGC 346 with the HST
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5 Stellar evolution overview
3. Main evolution phases
Dying stars
R Sculptoris (Red giant) with ALMA
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5 Stellar evolution overview
3. Main evolution phases
Pre-planetary nebula
Egg nebula with the HST
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5 Stellar evolution overview
3. Main evolution phases
Planetary nebula
Helix nebula with the HST
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5 Stellar evolution overview
3. Main evolution phases
Planetary nebula
Helix nebula (artist view)
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