Astro 1050 Wed. Oct. 30, 2002
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Today: Review Exam |
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Start Ch. 11: Neutron Stars
& Black
Holes |
Exam #2 Results
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Results posted on WebCT |
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Mean was a 58. Curve is 17 points. |
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Too long (esp. with the table
questions)? |
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Solar evolution on HR diagram important |
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See me if you have questions about the
grading |
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Will be included on prelim grades |
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Lab is 25% and will help grades |
The Crab Nebula –
Supernova from 1050 AD
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Can see expansion between 1973 and 2001 |
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Kitt Peak National Observatory Images |
What happens to the
collapsing core?
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Neutron star (more in next chapter) |
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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 |
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Binary Pulsars |
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Gamma Ray Bursts? |
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Pulsar Planets |
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Black Holes -- basics
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Nothing we know of can stop collapse
after neutron pressure fails |
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Consider escape velocity from the
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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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. |
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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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 will 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 |