Black Holes

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Black Holes

• What is a black hole?
• Do BHs exist in Nature?
• YES!
• How do we find them?
• What do they look like?

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Stars Pressure Balances Gravity
From www.astronomynotes.com
The Sun
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Eluding Gravitys Grasp

Escape Velocity
Escape Velocity Speed Needed To Escape An
Objects Gravitational Pull
Mass M Radius R
Earth Vesc 27,000 miles/hour (11 km/s) Sun
Vesc 1.4 million miles/hour (600 km/s)
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Dark Stars Rev. John Michell (1783)
Pierre-Simon Laplace (1796)

• What if a star were so small, escape speed ?
  speed of light?
• A star we couldnt see!

Earth mass R ? 1 inch Solar mass R ? 2
miles
Vesc speed of light ?
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1915 General Relativity, Einsteins Theory of
Gravity 1916 Schwarzschilds Discovery of BHs
in GRBHs only understood accepted in the
1960s (Term Black Hole coined by John Wheeler
in 1967)
Albert Einstein
Karl Schwarzschild
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Black Holes in GR
If an object is small enough, gravity overwhelms
pressure and the object collapses. Gravity is so
strong that nothing, not even light, can escape.
Radius of a BH ? 2 miles for a solar
mass ? 1 inch for an Earth mass NOT a solid
surface All Mass at the Center
(GR not valid there)
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Black Holes
The neutron star limit is 3 Msun A collapsing
stellar core that weighs gt3 Msun becomes a black
hole. Its radius is less than 2GM/c2, the
Schwarzschild radius.
The gravitational potential energy released
during the collapse rises the stars temperature
and pressure.
Since Emc2, energy must exert some gravitational
attraction. In a very massive and tiny core, the
energy acts like additional mass, helping the
collapse.
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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).

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Where are BHs Found?
Centers of Galaxies
Binary Stars
1 BIG BH per galaxy million-billion x mass of
sun formation not fully understood
millions of little BHs per galaxy 10 x mass
of sun formed by collapse of a massive star
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How do we know its a BH?

• Nature is tricky couldnt it be another small
  star like a neutron star or a
  white dwarf?
• Measure mass of X-ray star by motion of its
  companion (a star like the sun)
• Mass gt 3 solar
• masses ? BH!
• Roughly a dozen BHs found this way (tip of the
  iceberg)

Chandrasekhar
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Understanding Black Holes
The radius of the event horizon is called the
Schwarzschild radius. For a 10 Msun black hole,
it is 30 km.
Effects of gravity on space-time
The theory of relativity predicts that time
should run more slowly as the force of gravity
grows stronger. The light coming out of a strong
gravitational field should show a gravitational
redshift. A body falling through the event
horizon will be stretched and squeezed.
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Evidence for Black Holes
The center of a black hole is called a
singularity. This is where all the black hole
mass resides.
Nothing can escape through the event
horizon. Black holes can only be detected
indirectly. They can be companions in binary
systems.
The most promising galactic candidate to black
holes is called Cygnus X-1. This binary contains
a 18Msun star and an unseen 10 Msun object.
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Stars in the Central Light-Year of the Galaxy
Keep Zooming In
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The Galactic Center
The galactic center is located in the
constellation Sagittarius and is not seen in the
visible sky.
We can detect it in the infrared and radio region.
There is a bright radio source in the center
(Sagittarius A), a lot of gas clouds, and a
large star cluster.
Motions in Sgr A indicate that it contains a few
million solar masses in 3 light years across.
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The Moral of the Story

• Physicists said that Black Holes could exist
• the ultimate victory of gravity over all other
  forces
• Astronomers find that BHs do exist
• 1 Big BH per galaxy ( million-billion solar
  masses)
• millions of little BHs per galaxy ( solar
  mass)
• BHs are responsible for the most dramatic and
  energetic phenomena in the universe
• BHs are seen via the light produced by
  infalling gas via the gravitational
  pull that they exert on nearby objects