Main sequence stars with mass greater than about 8 solar masses
up as Type II supernovae and leave
behind a core which is too massive to
be a neutron star.
It is believed than for these objects the core
collapses into a singularity
Black hole: escape velocity > the speed
General Relativity vs.
Special Relativity (Einstein, 1905)
Nothing travels faster than the speed
These properties are manifestations of GENERAL RELATIVITY (Einstein's
Theory of gravity near very dense objects like black holes).
RELATIVITY is the physics of objects with velocities near
the speed of light = c (1 billion km/hour)
velocities add differently than simply adding
If you look at something going close to the speed of light,
time dilates (clocks appear to be running slow)
lengths contract (rulers appear to be squished along the direction of
For v<<c the
equations of special relativity predict very small corrections to
Newtonian gravity, which is why we don't notice these effects in
However, in fact this is only a two dimensional
depiction of what is
really a 3 dimensional object.
Schwarzchild Black Hole
Karl Schwarzchild (1916) investigated the structure of a black hole,
solving the equations of General Relativity.
Singularity: The star's
mass has compressed to infinite density at the singularity
Event Horizon: Within the
event horizon, no light or anything else can escape
Photon Sphere: Photons
can get trapped here, and just orbit the event horizon -- they don't
fall in and they don't escape
Kerr Black Hole
New Zealand astronomer Roy Kerr described a rotating black hole in 1963
We expect that as
material accretes onto the black hole, it brings angular momentum,
causing the black hole to "spin" ever faster
Here the singularity becomes a ring.
Within the ring, space may be curved like this:
Gravity is REPULSIVE, not ATTRACTIVE!
If a Kerr black hole gets spinning fast enough, the
speed of rotation at the edge of the singularity approaches the speed
of light. When it reaches the speed of light, the ergosphere
disappears, and what's left is a "naked
In science fiction stories, people talk about
wormholes, which are naked singularities which are connected to
"parallel universes" and would allow time travel faster than the speed
Falling into a black hole:
will be very severe next to a black hole. (The force of gravity
at your head is much less than the force of gravity at your toes).
Time slows down and an external observer thinks you never reach the
black hole horizon.
Stephen Hawking (author of A Brief History of Time
and other popular
books) postulated that the following process happens:
Particle-antiparticle pairs are sometimes created
outside the event horizon
of a black hole.
Three things can happen to a pair of particles just
the event horizon:
Both particles are pulled into the black hole.
Both particles escape from the black hole.
One particle escapes while the other is pulled
into the black hole.
For the third possibility, the particle that has
escaped becomes real
and can therefore be observed from Earth. The particle that was pulled
the black hole remains virtual and must restore its conservation of
by giving itself a negative mass-energy. The black hole absorbs this
mass-energy and as a result, loses mass and appears to shrink.
This may make black holes EVAPORATE, ie if you wait
long enough an isolated
black hole will lose enough mass and no longer exist.
EVIDENCE FOR STELLAR BLACK HOLES:
There appear to be an estimated 10 million stellar
black holes in the Milky Way galaxy.
We see these black holes as X-ray sources, because
of the very hot accretion
disk which forms, as material "accretes" onto the black hole
from a companion star.
The black hole masses are 1-10 solar masses, and the
event horizons are a few kilometers across.
X-ray Binary stars: Cygnus
Why do we think Cygnus
X-1 is a black hole?
Cygnus X-1 is a very bright X-ray source,
on timescales less than about 0.01 second.
Thus, the X-ray emitting region is less than
across, about the size of the Moon.
Cyg X-1 has a stellar companion, and spectra of
the star can be
like any spectroscopic binary to yield a mass.
The mass of the unseen companion star is at
least 10 solar
The only way the companion can be that small and
if it is a black hole
More recently, models of the X-ray spectra have
been used to
that the spectra we see could only be produced by a black hole
accretion disk, not a neutron star accretion disk.
Another prediction of General Relativity which has been confirmed by
observations is the existance of GRAVITATIONAL RADIATION or GRAVITY
Just as electromagnetic waves (light) are produced by vibrating charges
(electrons), motions of masses can produce GRAVITATIONAL RADIATION, or
waves in the curvature of space, which travel through space at the
speed of light.
Strong gravitational radiation would be expected from, for example, two
black holes orbiting each other, or coalescing.
How would you detect GRAVITY WAVES? With a Gravity Telescope, of
Laser Interferometry Gravitational Wave Observatory
Two sites, one in
Hanford, Washington; the other in Baton Rouge, LA
Measure motions between two mirrors, separated by 2.5 miles:
LIGO has not found anything yet. LISA is a space version of LIGO
in the planning stages.
Meanwhile, astronomers found evidence for graviational waves:
In 1975, a binary pulsar, called PSR1913+16 was discovered. It
consists of two neutron stars in orbit around each other, separated by
a distance about equal to the radius of the Sun.
The orbital period of the two neutron stars around each other is
slowing down in time -- exactly as you would expect if the binary is
producing gravitational radiation and therefore losing energy.
The agreement with General Relativity predictions is better than 0.5%,
and is arguably the strongest evidence we have that General Relativity
In 1993, Joseph Taylor and Russell Hulse, who discovered the
binary pulsar, won the Nobel Prize in Physics.