astronomy 114astro.umass.edu/~weinberg/a114/lectures/lec20.pdf · 2007. 3. 28. · radius of white...
TRANSCRIPT
Astronomy 114
Lecture 20: Death of stars
Martin D. Weinberg
UMass/Astronomy Department
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—1/19
Announcements
PS#5 posted today (and available up-front)
Due next Wednesday: 4 Apr
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—2/19
Announcements
PS#5 posted today (and available up-front)
Due next Wednesday: 4 Apr
Today:
How stars evolvePlanetary nebulaWhite dwarfsHigh-mass evolutionSupernovae
Stellar Evolution, Chaps. 21 & 22
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—2/19
HR diagram
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—3/19
Thermal pulsations
Fusion in shell around the core only createsvariability
More difficult heating through contraction to regulatepressure
Tends to overshoot
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—4/19
Envelope ejection
Core continues to contract
H and He burning in shell
Envelope pulsates andeventually becomesunbound from the core!
Envelope forms a“planetary nebula” aroundcore
Core moves to the left onHR diagram
White Dwarf
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—5/19
Planetary nebula
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—6/19
Planetary nebula
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—6/19
Low-mass evolution: quick review
Core H exhausted, H shellburning
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—7/19
Low-mass evolution: quick review
Core H exhausted, H shellburning
Helium flash, He coreburning
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—7/19
Low-mass evolution: quick review
Core H exhausted, H shellburning
Helium flash, He coreburning
Core He exhausted, Heand H shell burning
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—7/19
Low-mass evolution: quick review
Core H exhausted, H shellburning
Helium flash, He coreburning
Core He exhausted, Heand H shell burning
Pulsation, envelopeejection
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—7/19
Low-mass evolution: quick review
Core H exhausted, H shellburning
Helium flash, He coreburning
Core He exhausted, Heand H shell burning
Pulsation, envelopeejection
Core moves to the left onHR diagram⇒White Dwarf
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—7/19
White dwarf stars
Cores of low-mass stars collapse after losing theirenvelopes
Generate luminosity (Kelvin-Helmholtz mechanism)
R = 0.01R⊙ ≈ REarth!
At these densities, laws of Quantum Mechanicsbecome imporant, as in Hydrogen atom
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—8/19
Pauli exclusion principle
Two identical electrons, located in the same region ofspace, cannot have the same velocity
All electrons must have different trajectories
The electrons are packed so tightly that some of theelectrons are forced to have high velocities ⇒ highpressure
Pressure depends only on density and isindependent of temperature!
Pressure of electrons remains constant as thetemperature falls or rises
Helium flash!
Teaspoon of white dwarf stuff weighs as much asSUV!
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—9/19
Maximum mass of a white dwarf
White dwarfs with larger masses have smaller radii
The pressure within a white dwarf depends only ondensity, not on temperature, so larger density isneeded to balance gravity
At M = 1.4M⊙, degeneracy pressure can no longerprovide enough pressure
Radius of white dwarf approaches zero, in otherwords, a white dwarf can not be more massive than1.4M⊙
Known as: Chandrasekhar limit
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—10/19
Implication for stellar evolution
AGB stars lose mass in winds and envelope ejection
Star with initial mass M = 4M⊙ looses 2.6M⊙
Leaves 1.4M⊙ remnant
Star with M > 4M⊙ will not loose enough mass tohave a remant white dwarf above the Chandrasekharlimit
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—11/19
Evolution of High-Mass Stars
Stars with mass greater than 4 solar masses:Burn hot, Live fast, Die young
Burns H to He in core
Builds up a He core in roughly 10 million years
Giant phase: huge envelope, R = 5 AU!
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—12/19
Evolution of High-Mass Stars
Stars with mass greater than 4 solar masses:Burn hot, Live fast, Die young
Burns H to He in core
Builds up a He core in roughly 10 million years
Giant phase: huge envelope, R = 5 AU!
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—12/19
High-Mass Helium Fusion
Helium burning ignites in core
Hydrogen burning in shell
Build up of Carbon (and Oxygen) core
Star becomes a blue supergiant
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—13/19
High-Mass Helium Fusion
Helium burning ignites in core
Hydrogen burning in shell
Build up of Carbon (and Oxygen) core
Star becomes a blue supergiant
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—13/19
Carbon burning
He runs out in core
He and H burning in shells
Becomes a red supergiant (again)
Core collapses and ignites Carbon burning
Tcore > 6 × 108 K!
Density > 1.5 × 105 g/cc
This phase lasts for about 1000 years,then Carbon exhausted in core
Leaves: oxygen, neon, magnesium core
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Nearly the end . . .
White dwarf for stars with M < 8M⊙
Neon burning: few years
Oxygen burning: 1 year
Silicon burning: 1 day
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—15/19
Nearly the end . . .
White dwarf for stars with M < 8M⊙
Neon burning: few years
Oxygen burning: 1 year
Silicon burning: 1 day
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—15/19
The very end: iron core
Star after Silicon burning
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—16/19
The Core of an Evolved Star
An element factory!
An "onion skin" of different elements
Iron core - if the star is massive enough
Origin of commonly found terrestrial elements
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—17/19
The Core of an Evolved Star
An element factory!
An "onion skin" of different elements
Iron core - if the star is massive enough
Origin of commonly found terrestrial elements
The end is catastrophic!
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—17/19
What next?
The most stable elementis Iron (26Fe
56)
Need energy to split upFe or to add to Fe.
For elements lighterthan iron:
Fusion releasesenergy
For elements heavier than iron:
Fission releases energy
The universe is slowly turning to iron!
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—18/19
What next?
The most stable elementis Iron (26Fe
56)
Need energy to split upFe or to add to Fe.
For elements lighterthan iron:
Fusion releasesenergy
For elements heavier than iron:
Fission releases energy
The universe is slowly turning to iron!
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—18/19
Three special cases
1. White Dwarf stars
2. Pulsating stars
3. Supernovae
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—19/19
Three special cases
1. White Dwarf stars
2. Pulsating stars
3. Supernovae
4. Neutron stars
5. Black holes
A114: Lecture 20—28 Mar 2007 Read: Ch. 22,23 Astronomy 114—19/19