ASTR 400/700:
Stellar Astrophysics
Stephen Kane
Upcoming schedule
●
Nov 17: Stellar remnants (Chapter 17)
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Nov 29: Jonathan Fortney visit
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Dec 1: Summary
A700 Oral Exam
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Dec 6:
- Archana Dobaria
“Stellar Formation in Different Galaxies”
- Yuzo Ishikawa
“Understanding the Properties and Formation of
Black Holes”
●
Dec 8:
- Daniel McKeown
“Stellar Content in the Illustris Simulation”
- Heechan Yuk
“Structures and Mechanisms of Supernovae”
Stellar Remnants
Chapters 16 & 17
The Remnants of Sun-Like Stars:
White Dwarfs
First example:
Sirius B (Astrometric
binary; discovered 1862)
M ≈ 1 M0
L ≈ 0.03 L0
Te ≈ 27,000 K
⇒ R ≈ 0.008 R0
⇒ ρ ≈ 3x106 g/cm3
Classes of White Dwarfs
• DA White Dwarfs:
–
–
Pressure broadened Hydrogen
absorption lines in spectrum
Largest group. About 2/3.
• DB White Dwarfs:
–
–
–
Hydrogen lines absent
Helium absorption lines
About 8% of sample
• DC White Dwarfs:
–
–
No lines. Only continuum devoid of
features
About 14%
• DQ White Dwarfs:
–
Carbon features in spectra
• DZ White Dwarfs:
–
Evidence of metal lines
White Dwarfs
Degenerate stellar remnant (C,O core)
Extremely dense:
1 teaspoon of WD material:
mass ≈ 16 tons!!!
Chunk of WD material the
size of a beach ball would
outweigh an ocean liner!
Central pressure:
Pc ~ 3.8*1022 N/m2 ~ 1.5x106 Pc,0
for Sirius B
The Chandrasekhar Limit
The more massive a white dwarf, the smaller it is.
RWD ~ MWD-1/3 => MWD VWD = const. (non-rel.)
WDs with more than ~ 1.44 solar masses can
not exist!
Transition to relativistic
degeneracy
White dwarf
supernovae
can also be
used as
standard
candles.
Cooling Curve of a White Dwarf
Nuclei settling in a
crystalline structure,
releasing excess potential
energy
Neutron Stars
Radial Structure of a Neutron Star
- Heavy Nuclei (56Fe)
- Heavy Nuclei (118Kr); free neutrons;
relativistic, degenerate e-
- Superfluid neutrons
Properties of Neutron Stars
Typical size: R ~ 10 km
Mass: M ~ 1.4 – 3 Msun
Density: ρ ~ 4x1014 g/cm3
→ 1 teaspoon full of NS matter has a mass
of ~ 2 billion tons!!!
Rotation periods: ~ a few ms – a few s
Magnetic fields: B ~ 108 – 1015 G
(Atoll sources; ms
pulsars)
(magnetars)
The Lighthouse Model of Pulsars
A Pulsar’s magnetic field has a dipole
structure, just like Earth.
Radiation is emitted mostly
along the magnetic poles.
Rapid rotation along axis not aligned
with magnetic field axis
→ Light house model of pulsars
Pulses are not perfectly regular
→ gradual build-up of average pulse profiles
Why Pulsars Must Be Neutron Stars
Circumference of Neutron Star = 2π (radius) ~ 60 km
Spin Rate of Fast Pulsars ~ 1000 cycles per second
Surface Rotation Velocity
~ 60,000 km/s
~ 20% speed of light
~ escape velocity from NS
Anything else would be torn to pieces!
Neutron Star Limit
• Quantum mechanics says that neutrons in the
same place cannot be in the same state.
• Neutron degeneracy pressure can no longer
support a neutron star against gravity if its mass
exceeds about 3MSun.
Black Holes
• In 1783 John Mitchell
pondered that the escape
velocity from the surface
of a star 500 times larger
than the sun with the
same average density
would equal the speed of
light.
vesc = 2GM /r = 2G(500M ⊗ ) /7.93R⊗ = c
In 1939 J. Robert Oppenheimer and Hartland
Snyder described the ultimate gravitational
collapse of a massive star that has exhausted
its sources of nuclear fusion. They pondered
what happened to the cores of stars whose
mass exceeded the limit of neutron stars..
• R=2.95(M/Msun) km!
In 1967 the term “black hole” was coined
By John Archibald Wheeler
If the Sun shrank into a
black hole, its gravity
would be different only
near the event horizon.
Black holes don’t suck!