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General Astronomy
Black Holes
A huge great enormous thing, like — like nothing. A
huge big — well, like a — I don’t know — like an
enormous big nothing ...
Piglet describes the Heffalump,
in Winnie the Pooh by A.A. Milne
Black Holes
• After a massive star supernova, if the
core has a mass > 3 M, the force of
gravity will be too strong for even neutron
degeneracy to stop.
• The core will collapse out of existence.
GRAVITY FINALLY WINS!!
• This is what we call a black hole.
• The core becomes infinitely small.
• Since 3 M or more are compressed into
an infinitely small space, the gravity of the
core is HUGE!
SuperNova
Black Holes
Several months after Einstein presented the General
Theory of Relativity in 1915, a young German soldier
serving at the Russian front, Karl Schwarzschild (18731916), solved the equations and described a black hole.
This results in the notion of an Event Horizon with a
Schwarzschild radius.
Karl Schwarzschild (1876-1916)
“Size” of a Black Hole
• Spacetime is so highly warped around a black hole, even
light can not escape.
• Schwarzschild Radius – the distance from a black hole
where the escape velocity equals the speed of light.
Rs = 2GM/c2 (Rs in m; M in kg)
• A sphere of radius Rs around the black hole is called
the event horizon.
Structure of a black hole
A nonrotating black hole has only a
“center” and a “surface”
• The black hole is
surrounded by an event
horizon which is the
sphere from which light
cannot escape
• The distance between the
black hole and its event
horizon is the
Schwarzschild radius
(RSch= 2GM/c2)
• The center of the black
hole is a point of infinite
density and zero volume,
called a singularity
Bending of Light Path Around Black Holes
At a distance of about
1.5 Rsch of a black hole,
spacetime is distorted
so much that photons
emitted from the back
of your head actually go
around the black hole
and come back to you.
Photon orbits around a black hole
Radius of the Event Horizon
For example, suppose the Earth could be
compressed enough to become a blackhole
To be a blackhole, Earth would have to be compressed to
about the size of a dime – slightly less than 1 centimeter
Rotating black holes
• A rotating black hole
(one with angular
momentum) has an
ergosphere around
the outside of the
event horizon
• In the ergosphere,
space and time
themselves are
dragged along with
the rotation of the
black hole
Schwarzschild Radii
Object
Mass
Radius
Atom
10-20 Kg
10-51 cm
Human Being
70 Kg
10-23 cm
Earth
6.0 x 1024 Kg
0.89 cm
Sun
2.0 x 1030 Kg
3.0 Km
Galaxy
1011 M
10-2 Ly
Universe
1023 M
1010 Ly
Singularities
Singularity Theorem:
Every black hole must have a singularity inside
itself.
A naked singularity is not inside a black hole (not
surrounded by an event horizon), and therefore
can be seen by someone outside it.
Cosmic Censorship Theorem:
The laws of physics prevent naked singularities
from forming when a star collapses.
Most properties of
matter vanish when
matter enters a black
hole, such as chemical
composition, texture,
color, shape, size,
distinctions between
protons and electrons,
etc.
Only 3 parameters remain…
• Mass
– As measured by the black hole’s effect on
orbiting bodies, such as another star
• Total Electric Charge
– As measured by the strength of the
electric force
• Spin (angular momentum)
– How fast the black hole is spinning
Types of black holes
• Schwarzschild (1916)
– mass
• Reissner-Nordström (1916, 1918)
– mass, electric charge
• Kerr (1963)
– mass, angular momentum
• Kerr-Newman (1965)
– mass, angular momentum,
electric charge
Sizes
• Black Holes Come in Varying Sizes:
– “Stellar Mass”
• 5 - 20 times the mass of the sun
• Result from supernova explosion of massive
star
– Massive
• Millions times the mass of the sun
• Lie in centers of galaxies
Falling into a black hole
Falling into a black hole gravitational tidal forces
pull spacetime in such a way that time becomes
infinitely long (as viewed by distant observer).
The falling observer sees ordinary free fall in a
finite time.
Falling into a black holes
•
•
•
With a sufficiently large black hole, a freely
falling astronaut would pass right through the
event horizon in a finite time, would be not
feel the event horizon.
A distant observer watching the freely falling
observer would never see her fall through the
event horizon (takes an infinite time).
Falling into smaller black hole, the freely
falling observer would be ripped apart by tidal
effects.
Falling into a black hole
• Signals sent from the freely falling observer
would be time dilated and redshifted.
• Once inside the event horizon, no
communication with the universe outside the
event horizon is possible.
• But incoming signals from external world can
enter.
Falling into a black
hole:
time dilation
Falling into a black hole
• As matter approaches the event horizon…
– the tidal forces are tremendous
– the object would be “spaghettified”
Spaghettification!
Falling into a black hole
• The tidal force between head and toes is now 1
million g, for a 30 solar mass black hole.
• The tides wouldn't be so bad for a very
massive black hole. The tide at 1
Schwarzschild radius would be less than 1g if
the black hole exceeded 30,000 solar masses.
• From the event horizon to the central
singularity will take 0.0001 seconds in free
fall, for a 30 solar mass black hole. (Although
to the observer it would take forever.)
• The infall time is proportional to the mass of
the black hole.
Orbiting a Black Hole
Black Holes: Do They Really Exist?
We cannot see black holes directly, so we have to look for indirect evidence…
What would you look for to find a stellar-massed black hole, like those
formed after the death of high mass stars?
S. Harris
“Seeing” a Blackhole
If no light can escape from a black hole,
how can we know that they are there?
You can’t see one directly, but we can see
what is happening to the accretion disk
surrounding the event horizon.
Seeing black holes
Optical
• Material swirls around
central black hole.
• Gas near black hole
heats up to UV and X-ray
temperatures.
• This heats surrounding
gas, which glows in the
optical.
Binary Star Systems
• Black holes are often part
of a binary star system,
two stars revolving around
each other.
• What we see from Earth is
a visible star orbiting
around what appears to be
nothing.
• We can infer the mass of
the black hole by the way
the visible star is orbiting
around it.
• The larger the black hole,
the greater the
gravitational pull, and the
greater the effect on the
visible star.
Chandra illustration
The Case of Cygnus X-1
Cygnus X-1 is a X-ray binary system with a bright star of
18 M⊙, and an unseen (invisible in the visible) companion
of about 10 M⊙. If the mass estimate is of the X-ray
source is correct, than it certainly exceeds the upper
mass limit of neutron star, making it a prime stellar
black hole candidate.
Accretion Disks
Gas from the companion star is drawn by
gravity onto the black hole in a swirling
pattern. As the gas nears the event
horizon, a strong gravitational redshift
makes it appear redder and dimmer.
When the gas finally crosses the event
horizon, it disappears from view.
Because a black hole has no surface, the
central region is black.
As the gas approaches the neutron star,
a similar gravitational redshift makes the
gas appear redder and dimmer. However,
when the gas strikes the solid surface of
the neutron star, it glows brightly.
Rotating Blackholes
The orbit of a particle near a black hole depends on the curvature of space around
the black hole, which also depends on how fast the black hole is spinning. A spinning
black hole drags space around with it and allows atoms to orbit nearer to the black
hole than is possible for a non-spinning black hole. The tighter orbit means stronger
gravitational effects, which means that more of the X-rays from iron atoms are
shifted to lower energies.
X-rays
• Black holes capture
nearby stellar material.
• As the gas gets closer
to the black hole, it
heats up.
• Gas heats to
temperatures in the
range of millions of
degrees.
• Gas heated to these
temperatures releases
tremendous amounts of
energy in the form of
X-rays.
XMM-Newton Illustration
Seeing black holes
X-ray: Frame Dragging
Detection of a period
in GRO J1655-40 due
to precession of the
disk.
This precession
period matches that
expected for frame
dragging of spacetime around the black
hole.
Credit: J. Bergeron, Sky & Telescope Magazine
X-ray: Jets
Cen A is known to be a peculiar
galaxy with strong radio emission.
Optical image of Cen A
But it is also a strong X-ray
emitter, and has an X-ray jet.
Chandra image of Cen A
SS 433
The observation is of a familiar source
named SS 433 -- a binary star system
within our Galaxy in the constellation
Aquila, the Eagle, about 16,000 light years
away.
The black hole and its companion are
about two-thirds closer to each other than
the planet Mercury is to the Sun.
The jets shoot off at 175 million miles per
hour, 26 percent of light speed.
X-rays from Black Holes
In close binary systems, material flows from normal star to
black hole. X-rays are emitted from disk of hot gas swirling
around the black hole.
What’s Happenin’
Magnetic field from surrounding disk funnels
material into the jet
RX J1242-11
The first strong evidence of a
supermassive black hole ripping
apart a star and consuming a
portion of it.
Chandra detected a powerful X-ray
outburst from the center of the
galaxy RX J1242-11.
This outburst, one of the most
extreme ever detected in a galaxy,
was caused by gas from the
destroyed star that was heated to
millions of degrees Celsius before
being swallowed by the black hole.
Radio Jets from Black Holes
• Many black holes emit jets.
– Material in jet moving at 0.9c.
– Jet likely composed of electrons and
positrons.
• Magnetic fields surrounding black hole
expel material and form the jet.
– Interaction of jet material with magnetic
field gives rise to Radio emission.
M87 - An Elliptical Galaxy
With a curious feature
Radio shows the origin of the Jet
Sagittarius
*
A
If the black hole at the
center of the Milky Galaxy
is about 3 million solar
masses, then its size must
be smaller than 3 million
km, or 10 light seconds.
This is only a tiny spot near
Sagittarius A* in the
picture on the right.
The star approached the central Black Hole to within 17 light-hours - only
three times the distance between the Sun and planet Pluto - while traveling
at no less than 5000 km/sec
Sagittarius
*
A
Chandra image of central ~ 3 pc
IR (VLT) image of center pc
Baganoff et al.
Genzel et al.
Young cluster of massive stars
in the central pc loses ~ 10-3 M
yr-1 (  2-10" from BH)
1" = 0.04 pc  105 RS @ GC
Hot x-ray emitting gas
(T = 1-2 keV; n = 100 cm-3)
produced via shocked
stellar winds
Is it a Black Hole?
Possessing a large amount of mass does not make you qualify
for a black hole, you need to pack these masses into a space
smaller than the Schwarzchild radius of that mass…
More Black Holes Headline News…
RELEASE: 02-222
NEVER BEFORE SEEN: TWO
SUPERMASSIVE BLACK HOLES IN
SAME GALAXY
For the first time, scientists have proof
two supermassive black holes exist
together in the same galaxy, thanks to data
from NASA's Chandra X-ray Observatory.
These black holes are orbiting each other
and will merge several hundred million
years from now, to create an even larger
black hole resulting in a catastrophic event
that will unleash intense radiation and
gravitational waves…
Blackhole Candidates
Object
Location
Companion Star
Orbital Period
Mass of Compact Object
Cygnus X-1
Cygnus
O Supergiant
5.6 days
10-15 Solar masses
LMC X-3
Dorado
B3 main-sequence
1.7 days
4-11 solar masses
V616 Mon
Monoceris
K main-sequence
7.75 hours
3.3-4.2 solar masses
V404 Cygni
Cygnus
K main-sequence
6.47 days
8-15 solar masses
J1655-40
Scorpius
F-G main-sequence
2.61 days
4-5.2 solar masses
QZ Vul
Vulpecula
K main-sequence
8 hours
5-14 solar masses