# Black Holes

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```							Black Holes
Escape Velocity
The speed
needed to escape
from the
gravitational
field of a planet
(or other object)
Escape Velocity
2GM
v
R
 Depends on:
– Mass of the planet/object
What if the planet/object is
really heavy?
 The
escape velocity increases with the
mass of the planet/object.

 Can an planet/object be so heavy that
the escape velocity is greater than c?
The Speed of Light

   1675: Olaus Roemer
(Danish astronomer)
measures by timing
“transits” of
Jupiter’s moons

Kaufmann, Universe, 4th ed., p.80

c  300,000 km
s
The Idea of a Black Hole
John Michell (1783) , Pierre-Simon Laplace (1796):

 Question: What happens if the escape speed
from an object is greater than the speed of
light?
 Answer: If light consists of particles of matter,
they would not be able to escape

“dark star” ?
If light cannot escape from an
object it would appear black!
 Rearrange the escape
2GM
velocity equation to     Rs     2
determine how big a            c
black hole would be.

1915: General Relativity, Einstein’s Theory of Gravity

 Gravity is the bending
of spacetime
 Planets and stars cause
the “fabric” of the
universe to stretch

Albert Einstein
1916: Schwarzschild shows that Black Holes can be
produced in General Relativity (GR)

I had not expected that one could     Karl Schwarzschild
formulate the exact solution of the
problem in such a simple way.
—Albert Einstein
BHs only understood & accepted in the 1960s
(Term “Black Hole” coined by John Wheeler in 1967)

John Wheeler            Stephen Hawking
How small would our Sun have
to be to act like a Black Hole?

   M = 2.0 x 1030 kg

   RS  3 km
How small would our Earth have
to be to act like a Black Hole?

   M = 6.0 x 1024 kg

   RS  1 cm
How do Black Holes form?
 Stable stars have two
forces in equilibrium
 GRAVITY pulling in
 PRESSURE pushing out
 A Black Hole forms
when gravity crushes the
mass into a tiny volume
Will all stars form Black Holes?
 Low mass stars (<1.4 Msun)
will form White Dwarf stars
 Medium mass stars (<3 Msun)
will form Neutron stars
 Large mass stars (> 3 Msun)
will become a Black Hole.
Low Mass Stars
   If the mass of the star is < 1.4 Msun, it will
become a White Dwarf star.
Medium Mass Stars
 If the mass of the
star is < 3 Msun, it
will become a
Neutron star.
 One teaspoon of
this star would
weigh 100 million
tons!!
Large Mass Stars
 If the mass of the star is > 3 Msun, it will
become a Black Hole.
 The gravity is so
strong that it will
crush all the mass
into a single point!
Do we have any proof?
   YES, but since they don’t emit light we
cannot see Black Holes we must rely on
indirect observations.
X-ray sources
 As material gets sucked into a Black Hole it
will ‘scream’ and emit x-rays.
 Astronomers have located some very strong
x-ray sources.
 Computers can be used to visualize what
happens, using real data in the non-visible
spectrum.
http://hubblesite.org/newscenter/archive/releases/2001/03/video/a/

Computer visualization of data from the Hubble Space Telescope
Gravitational effects
   Black holes are heavy, so they will cause
other visible objects to act strange

http://www.eso.org/public/outreach/press-rel/pr-2002/video/vid-02-02.mpg

Images taken by European Space Observatory
http://www.stsci.edu/~marel/m32anim.html
Gravitational lensing
   Black holes are heavy, so they will make
light bend when it passes nearby.
Animation of black hole passing in front of deep space image
Other Black Hole Info

Time stops inside a black hole
Other Black Hole Info
At the centre of a black hole is a SINGULARITY,
much like the one that would have occurred during
BIG BANG
Other Black Hole Info

Black holes actually ‘glow’ and evaporate
Dispelling the Myths …
BHs are not cosmic vacuum
cleaners: only inside the horizon
is matter pulled inexorably inward

Far away from a BH, gravity
is no different than for any
other object with the same mass

If a BH were to replace the sun, the orbits of planets, asteroids,
moons, etc., would be unchanged
(though it would get really really cold).
Check out the following sites!!
http://hubblesite.org/explore_astronomy/black_holes/
http://www.thinktechnologies.com/portfolio/demos/Blackhole.swf

```
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