Download Chapter 12: Stellar Evolution - Otto

Chapter 12
Stellar Evolution
Infrared Image of Helix Nebula
Mass and Stellar Fate
• Low mass stars end life quietly
• Massive stars end life violently
• Massive - more than 8X M
Core-hydrogen burning
• Main sequence stars fuse H into He
• On main sequence for over 90% of life
• Hydrostatic equilibrium - pressure and
gravity balance
Figure 12.1
Hydrostatic Equilibrium
Evolution of a sun-like star
• Stages 1 - 6 (pre - main sequence)
• Stage 7 - main sequence
• Stages 8 - 14 (post main sequence)
Stages 8 and 9
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Stage 8 - Subgiant branch
Stage 9 - Red Giant branch
H depleted at center, He core grows
Core pressure decreases, gravity doesn’t
He core contracts, H shell burning increases
Star’s radius increases, surface cools,
luminosity increases
Figure 12.2
Solar Composition
Change
Figure 12.3
Hydrogen Shell Burning
Figure 12.4
Red Giant on the H-R Diagram
Stage 10 - Helium Fusion
• Red Giant core contracts (no nuclear burning
there)
• Central temperature reaches 108 K
• Fusion of He starts abruptly - Helium flash for
a few hours
• Star re-adjusts over 100,000 years from
stage 9 to 10
• H and He burning with C core - horizontal
branch
Figure 12.5
Horizontal Branch
Figure 12.6
Helium Shell Burning
Stage 11 - Back to Giant Branch
• C core contracts (no nuclear burning
there)
• Gravitational heating
• H and He burning increases
• Radius and luminosity increases
Figure 12.7
Reascending the Giant Branch
Table 12.1
Evolution of a Sun-like Star
Figure 12.8L
G-Type Star Evolution
Figure 12.8R
G-Type Star Evolution
Death of a low mass star
• For solar mass star, core temperature
not high enough for C fusion
• Outer layers drift away into space
• Core contracts, heats up
• UV radiation ionizes surrounding gas
• Stage 12 - A planetary nebula
• (nothing to do with planets)
Figure 12.9
Planetary Nebulae
Other elements
• As red giant dies, other elements
created in core
• O, Ne, Mg
• Enrich interstellar medium as surface
layers ejected
Dense matter
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Carbon core shrinks and stabilizes
Core density 1010 kg/m3
1000 kg in one cm3
Pauli Exclusion Principle keeps free
electrons from getting any closer
together
• This is a different sort of pressure
Stage 13 - White Dwarf
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Red giant envelope recedes
C core becomes visible as a white dwarf
Approximately size of earth, 1/2 mass of sun
White-hot surface, but dim (small size)
Glow by stored heat, no nuclear reactions
Fades in time to a black dwarf - stage 14
Figure 12.10
White Dwarf on an H-R Diagram
Table 12.2
Sirius B – A Nearby White Dwarf
Figure 12.11
Sirius Binary System
Figure 12.12
Distant White Dwarfs
Novae
• Plural of nova
• Some white dwarfs become explosively
active
• Rapid increase in luminosity
Figure 12.13ab
Nova Herculis
a) March 1935
b) May 1935
Figure 12.13c
Nova
Nova explanation
• White dwarf in a binary
• Gravitation tears material from
companion, forming accretion disk
around white dwarf
• Material heats until H fuses
• Surface burning brief and violent
• Novae can be recurrent
Figure 12.14
Close Binary System
Figure 12.15
Nova Matter Ejection
Evolution of High-Mass Stars
• All main sequence stars move toward
red-giant phase
• More massive stars can fuse C and
other heavier elements
• Evolutionary tracks are more horizontal
• 4 M star can fuse C
• 15 M star can fuse C, O, Ne, Mg and
become a red supergiant
Figure 12.16
High-Mass Evolutionary Tracks
Evolution of 4 M star
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No He flash
Hot enough to fuse C
Can’t fuse beyond C
Ends as a white dwarf
Evolution of 15 M star
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Rapid evolution
Becomes red supergiant
Fuses H, He, C, O, Ne, Mg, Si
Inner core of iron
Figure 12.17
Heavy-Element Fusion
Figure 12.18
Mass Loss from Supergiants
Examples in Orion
• Rigel - blue supergiant
• 70 R, 50,000X luminosity of sun
• Originally 17 M
• Betelgeuse - red supergiant
• 10,000X luminosity of sun in visible light
• Originally 12 to 17 M
High mass fast evolution
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Consider 20 M star
Fuses H for 10 million y
Fuses He for 1 million y
Fuses C for 1000 y
Fuses O for one year
Fuses Si for one week
Fe core grows for less than a day
Death of high mass star - 1
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Fe fusion doesn’t produce energy
Pressure decreases at core
Gravitational collapse
Core temperature reaches nearly 10 billion K
High energy photons break nuclei into
protons and neutrons - photodisintegration
• Reduced pressure, accelerated collapse
Death of high mass star - 2
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Electrons + protons  neutrons and neutrinos
Density 1012 kg/m3
Neutrinos escape, taking away energy
Further collapse to 1015 kg/m3
Neutrons packed together slow further
collapse
• Overshoots to 1018 kg/m3, then rebounds
• Shock wave ejects overlying material into
space
• Core collapse supernova
Figure 12.19
Supernova 1987A
Table 12.3
End Points of Evolution for Stars of Different
Masses
Novae and Supernovae
• Nova - explosion on white dwarf surface in a
binary system
• Supernova - exploding high mass star
• Million times brighter than nova
• Billions of times brighter than sun
• Supernova in several months radiates as
much as our sun in 10 billion years
Types of Supernovae
• Type I - very little H
• Sharp rise in brightness, gradual fall
• Type II - H rich
• Plateau in light curve
• Roughly half Type I and half Type II
Figure 12.20
Supernova Light Curves
Type II Supernovae
• Core collapse as previously described
• Expanding layers of H and He
Type I Supernovae
• Accretion disk around white dwarf can nova
• Some material adds to white dwarf
• Below 1.4 M (Chandrasekhar mass),
electrons support white dwarf
• Above 1.4 M, white dwarf collapses
• Rapid heating, C suddenly fuses throughout
• Carbon-detonation supernova
• Also possible for two white dwarfs to merge
Figure 12.21
Two Types of Supernova
Supernovae summary
• Type I - carbon detonation of white dwarf
exceeding 1.4 M
• Type II - core collapse of massive star,
rebound and ejection of material
• All high mass stars Type II supernova
• Only some low mass stars Type I supernova
• Low mass stars much more common than
high mass
• Type I and II about equally likely
Figure 12.22
Supernova Remnants
Heavy Element Formation
• All H and most He is primordial
• Other elements produced through stellar
evolution
Stellar evolution in star clusters
• All stars the same age
• Snapshot at one time
Figure 12.23
Cluster Evolution on the H-R Diagram
Figure 12.24
Newborn Cluster H-R
Diagram
Figure 12.25
Young Cluster H-R
Diagram
Figure 12.26
Old Cluster H-R Diagram
Figure 12.27
Stellar Recycling