Transcript 14-black-holes

Black Holes
Chapter 14
Review
• What is the life cycle of a low mass star (<8 solar
masses when on the main sequence)?
• What is the life cycle of a high mass star (>8 solar
masses when on the main sequence)?
• After a supernova, what are the two fates of the
core of the star?
• What determines whether the core will be a
neutron star or a black hole?
Topics
• Black hole
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what is it?
why is it so hard to detect?
how do we detect it?
how big can it be?
how small is it?
How it begins
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A type II supernova is the explosion of a giant star.
The core collapses.
If M<3 solar masses, it will be a neutron star.
If M>3 solar masses, even a neutron gas cannot
withstand the large gravitational force.
• The core continues to collapse; its volume
decreases to zero.
• The density becomes infinite.
• Note: the mass is still finite!
Characteristics
• At the center of the black hole, the gravitational
force is infinite.
• Light inside a certain radius cannot escape the
gravitational force.
– Schwartzchild radius
– Imaginary surface at this radius is the event horizon
– Light inside this event horizon would orbit the black
hole.
– 3 solar mass black hole, R=9 km
Is it a cosmic vacuum cleaner?
• No, as long as you’re outside the
Schwartzchild radius.
• For example, if the Sun were a black hole of
the same mass, the Earth’s orbit would be
NO different.
What does a black hole look like?
Look very closely!
Black Hole
So how do we detect it?
• The same way you detect Aunt Edna - by the effect she has
on others.
• If black hole is part of a binary system with giant, it
accretes matter from the giant.
– as the matter accelerates, it radiates x-rays
– accretion disk
– it could be a neutron star, so we must measure mass; how do we do
that?
• Bends light that passes nearby
– gravitational lensing
• Nearby objects orbit with very high orbital velocities and
small periods
– centers of galaxies are likely to be very massive black holes
– so how do we know how fast something is moving?
What makes supermassive black
holes?
• Typically at centers of
galaxies
• blackholes gobble up
stars and even each
other
• M~ millions of solar
masses
• see press release