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Black Holes
The Science Behind The Science Fiction
Eliot Quataert (Berkeley Astronomy Dept)
Science Fiction
“A Journey That Begins
Where Everything Ends”
“Infinite Space,
Infinite Terror”
A Muse for Popular Culture
The Far Side
Angry Yankee’s Fan
Gary Larson
The giant Schilling vortex has
become the black hole of popular
culture, sucking in all images and
sound and allowing only soundbites
about The Great Pitcher’s courage
and legacy to escape.
“Suddenly, through forces not
yet fully understood, Darren
Belsky’s apartment became
the center of a new black
hole”
What is a black hole?
Do BHs exist in Nature?
– YES!
How do we find them?
What do they look like?
First, Something Simpler: Stars
Pressure Balances Gravity
From www.astronomynotes.com
The Sun
Eluding Gravity’s Grasp
Escape Velocity
2GM
Vesc 
R
Escape Velocity
Speed Needed To
Escape An Object’s
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)
“Dark Stars”
Rev. John Michell (1783) & Pierre-Simon Laplace (1796)
Speed of light  1 billion miles/hour (3x105 km/s)
What if a star were so small,
escape speed > speed of light?
A star we couldn’t see!
Vesc = speed
of light 
Earth mass:
Solar mass:
R  1 inch
R  2 miles
1915: General Relativity, Einstein’s Theory of Gravity
1916: Schwarzschild’s Discovery of BHs in GR
BHs only understood & accepted in the 1960s
(Term “Black Hole” coined by John Wheeler in 1967)
Albert Einstein
Karl Schwarzschild
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)
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).
How do we find BHs in Nature?
Sidney Harris
“It’s black, and it looks like a hole.
I’d say it’s a black hole.”
Where are BHs Found?
Centers of Galaxies
1 BIG BH per galaxy
million-billion x mass of sun
formation not fully understood
Binary Stars
millions of ‘little’ BHs per galaxy
~ 10 x mass of sun
formed by collapse of a massive star
Shedding Light on BHs: X-ray Binaries
Matsuda
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Gas falling into a
BH gets very hot
and emits lots of
radiation in X-rays
Accretion is how we
“see” a black hole
If two stars orbit close enough to each other, mass
gets pulled from one and falls (accretes) onto the
other. The smaller the target object, the faster the gas
moves and the hotter it gets.
How do we know it’s a BH?
Nature is tricky: couldn’t 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)
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
How do we know it’s a BH?
Nature is tricky: couldn’t 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 > 3 solar
masses  BH!
Chandrasekhar
Roughly a dozen BHs found this way (tip of the iceberg)
Where are BHs Found?
Centers of Galaxies
1 BIG BH per galaxy
million-billion x mass of sun
unclear how they form
Binary Stars
millions of ‘little’ BHs per galaxy
~ 10 x mass of sun
formed by collapse of a massive star
The Milky Way Galaxy: ~ 100,000 light-years across
Scale: Size of Solar System: 0.01 light-years
Typical Distance btw. Stars: 1 light-year
Central Black Hole Mass: 4 million Msun
Also ~ millions of 10 Msun BHs
4 106 Msun
Black Hole
Stars in the Central
Light-Year of the Galaxy
Keep Zooming In …
Evidence for a Big BH at the
center of our Galaxy
10 light-days  size of solar system
Genzel et al; also Ghez et al.
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Motion of stars
at the center
of the Milky
Way over the
past decade
Evidence for a Big BH at the
center of our Galaxy
Velocities & Orbits
of Stars  Mass
10 light-days  size of solar system
Genzel et al; also Ghez et al.
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
BH in our Galaxy
weighs in at
4 MILLION
SOLAR MASSES
Light From Gas Falling
Into the Black Hole
BH
Infrared Image
X-ray Solar
ImageFlare
Analogy:
Brightness of Central Black Hole
Many Varieties of Massive BHs
Our
Galaxy
“Active Galactic Nuclei”
radio image
The BH ejects beams (“jets”) of
matter & energy far outside its host
galaxy into the surrounding universe
The BH can outshine all of
the stars in its host galaxy!
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 &
the gravitational pull that they exert on nearby objects
via