Black%20Holes

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Big Black Holes
Liz Puchnarewicz Mullard Space Science
Laboratory University College London www.mssl.ucl.
ac.uk
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How can we see a black hole?
They must be fed
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Where are they?
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Essential ingredients

Escape velocity
V
R
Speed of light
300,000 km/s
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Escape velocity
R
V

for Earth, v 7 km/s
make Earth 1,850,000,000x smaller, v c and
the Earth is a black hole about 1cm
across
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A demonstration
1. Take your mass, eg. star of mass 10 Suns 2.
Compress it until the radius is 30 km or less
Within the Schwarzschild radius, even light
cannot escape
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Black hole recipes
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How do we know its really a black hole?
It must be very, very dense - all of its mass
must fit within its Schwarzschild radius
So we have to measure
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Weighing monsters
The Eddington Limit
Where inward force of gravity balances the
outward push of radiation on the surrounding
gas.
So a measurement of quasar luminosity gives the
minimum mass
mass
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Measuring the monster
Light travel time effects
Photons leave A first. Brightness spreads to B -
so light from B arrives a few days after A. But
how far has it travelled?
distance velocity x time We measure time, so
what is velocity? Dont know but it cant be
more than the speed of light. So distance must be
less than velocity x time
So the diameter must be less than a few light days
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More astrophysics
mass
volume
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The End
Liz Puchnarewicz Mullard Space Science
Laboratory University College London www.mssl.ucl.
ac.uk