Astro 1050 Mon. Mar. 28, 2005
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Today: Go over Homework on board |
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Ch. 11: Neutron Stars, Black Holes |
Chapter 11: Neutron Stars
and Black Holes
What happens to the collapsing core?
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Neutron star |
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Quantum rules also resist neutron
packing |
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Densities much higher than white dwarfs
allowed |
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R ~ 5 km r ~ 1014
gm/cm3 (similar to
nucleus) |
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M limit uncertain, ~2 or ~3 MSun before it
collapses |
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Spins very fast (by conservation of
angular momentum) |
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Trapped spinning magnetic field makes
it: |
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Act like a “lighthouse” beaming out E-M
radiation (radio, light) |
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pulsars |
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Accelerates nearby charged particles |
Spinning pulsar powers
the
Crab nebula
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Red:
Ha |
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Blue: “Synchrotron” emission from high speed
electrons trapped in magnetic field |
Another pic of the Crab,
Pulsar
Why a “pulsar?”
“Lighthouse” Model for
Pulsars
Another Neutron Star in a
SNR
Other cool stuff about
Neutron Stars
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Novel Dragon’s Egg by Robert L. Forward |
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Short Story “Neutron Star” by Larry
Niven (available at www.fictionwise.com) |
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Binary Pulsars |
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Tests of general relativity |
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Pulsar Planets |
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Variation in pulsar times indicates a
wobble from a planet in orbit around the pulsar. |
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Black Holes -- basics
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Nothing can stop collapse after neutron
pressure fails |
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Escape velocity from a surface at
radius R: |
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As R shrinks (but M is fixed), Vescape
gets larger and larger |
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At some point VEscape=
c (speed of light) |
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Happens at Schwarzschild radius: |
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Not even light can escape from within
this radius |
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Examples:
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The Schwarzschild Radius: |
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Mass in solar masses Rs
(km) |
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10 |
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3 |
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2 |
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1 |
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0.000003 (Earth) |
Examples:
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The Schwarzschild Radius: |
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Mass in solar masses Rs
(km) |
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10 30 |
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3 9 |
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2 6 |
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1 3 |
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0.000003 (Earth) 0.9 cm |
Black Holes -- details
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Remember – gravity is same as before,
away from mass |
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Black holes do NOT necessarily pull all
nearby material in |
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A planet orbiting a new black hole
would just keep on orbiting as before (assuming the ejected material or
radiated energy didn’t have an effect) |
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Any mass can potentially be made into a
black hole – if you can compress it to a size smaller than RS =
2GM/c2 |
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1 MSun: 3.0 km 106 MSun 3´106 km
1 MEarth 8.9 mm |
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Black Holes -- details
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If you do make material fall into a
black hole, material will be falling at close to the speed of light when it
reaches RS |
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If that falling gas collides with and
heats other gas before it reaches RS, then light from that hot
material (outside RS) can escape (important in quasars!). |
Black Holes – detection
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By definition – can’t see light from
black hole itself |
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Can see large amounts of energy
released by falling material just before it crosses RS |
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Can see motion of nearby objects caused
by gravity of black hole |
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Black Holes – detection
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Example: Like White Dwarf accretion
disk but w/ black hole instead |
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Gas from red giant companion spills
over towards black hole |
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Gas spirals in toward black hole,
through accretion disk |
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Gas will be much hotter because it
falls further, to very small RS |
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Gas will be moving at very high
velocity |
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Much faster than with white dwarf since
much closer (P2 µ a3) |
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Signature of black hole: Very high energy release, very high
velocity |
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We find MASSIVE black holes in centers
of most galaxies |
Cygnus X-1
More Cool Stuff About
Black Holes
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Time Dilation – originally “Frozen
Stars” |
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Gravitational Redshift |
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Wicked Tidal Forces |
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Hawking Radiation |