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Cliff Lau University of Toronto
AST201 Lecture 13 (March 3, 2011)
When a low-mass star like the Sun dies, it will become a white dwarf
When a massive star goes supernova, one of three things can happen:
1.The star could just blow itself completely apart, leaving nothing
2. It can blow its outer layers, leaving only the core of neutrons a
neutron star (diameters of about 10km size of Manhattan; a teaspoon
of neutron star matter would have the same mass as about 25,000 CN
Towers; neutron stars have powerful magnetic fields which channel
material to their poles, heating them up; due to conservation of angular
momentum, neutron stars can spin more than 30 times/s; it appears to
blink on and off as the star spins it pulses on and off so its called a
3.The star turns into a black hole
Higher density higher Vescape
Escape speeds from Earths surface is about 11km/s, if we compress Earth
until it was 1m in diameter the escape speed would be about 28,000km/s
Once the escape speed of the object reaches the speed of light, nothing can
ever escape from it it becomes a black hole
The boundary between the inside and outside of a black hole is called the
event horizon where Vescape = C
The radius of the event horizon (size of the black hole) is called the
Schwarzschild radius
Myth: black holes are comic vacuum cleaners
Truth: black holes have exactly as much mass as the things they formed from,
and so exactly as much gravity; they do not have special sucking power
Myth: black holes are inherently dangerous
Truth: the only dangerous thing about them is that, one youre inside, you
cant get out; but were you planning to go anywhere near that much mass
Myth: there is something at the event horizon
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