Download black hole

It was discovered in
the early 1990’s that
the pulse period of a
millisecond pulsar 500
parsecs from earth
varies in a regular way.
It fluctuates on two time
scales: 67 days and
98 days. This may be
caused by the Doppler
effect as the pulsar
wobbles in space.
This wobble may
be caused by the
combined gravitational
pull of two planets with
67 and 98 day orbital
periods.
This is the first
evidence for
planets outside
our solar system.
These planets were
probably not formed
the same way as
Earth because the
supernova would have
destroyed them.
Neutron stars rest
in an equilibrium:
gravity vs. the
pressure the
squeezed neutrons.
There is a situation
where the
gravitational pull is
too great for the
neutrons to withstand
the pressure.
Low mass stars
(less than 1.4 solar
masses) leave
behind a white dwarf
at the end of the
star’s life.
High mass stars
(between 1.4 and
3 solar masses)
produce a
neutron star.
Above a solar
mass of 3, even
the packed
neutrons cannot
hold up vs. gravity.
Once this pressure
is exceeded, no
force known can
counteract gravity.
The core collapses
forever.
Gravity around the
core is so great that
light can’t escape.
This object emits no
light, no radiation;
nothing escapes.
This is a black hole.
The density and the
gravitational field
become infinite. It is
called a singularity.
Two key facts from Einstein’s
relativity theories help us
understand black holes:
• 1. Nothing can travel faster
than light.
• 2. All things, including light,
are attracted by gravity.
Escape velocity is
inversely proportional
to a star’s radius.
The smaller an object
becomes, the higher
escape velocity
becomes.
Eventually escape
velocity can exceed the
speed of light. This
would occur if the Earth
was compressed to the
size of a grape.
A similar
compression
occurs in a black
hole, but with a
much larger
beginning mass.
The critical radius
at which escape
velocity equals c,
is the Schwarzchild
Radius.
The Schwarzchild radius is
proportional to the mass.
• For Earth’s mass it is 1 cm.
• For Jupiter’s mass it is 3 m.
• For the Sun’s mass it is 3 km.
• For 3 solar masses it is 9 km.
The Schwarzchild
radius is the radius
to which an object
would have to be
compressed to
become a black hole.
The surface of an
imaginary sphere with
radius equal to the
Schwarzchild radius and
centered on a collapsing
star is called the
event horizon.
The event horizon defines
the region within which
no event can be seen or
known by anyone outside.
It is the “surface” of a
black hole, but no matter
is associated with it.
A 1.4 solar mass neutron star
has a radius of 10 km and a
Schwarzchild radius of 4.2
km. Increasing a neutron
star’s mass increases its
Schwarzchild radius, but the
physical radius is unchanged.
When a neutron star’s mass
exceeds about 3 solar
masses, it surface would lie
just within its own event
horizon and the star would
collapse beyond the
Schwarzchild radius to a
point singularity.
So, if at least 3 solar
masses remain after
a supernova, the
remnant core will
collapse to a
black hole in less
than one second.
At 1.5 Schwarzchild
radii from the center of
the star, photons
emitted perpendicularly
travel in a spherical
orbit forming
a photon sphere.
Relativity states that matter
“warps” (curves) space in its
vicinity. The greater the mass,
the greater the warping. Close
to a black hole, the mass is so
great that space “folds over”
on itself causing objects within
to disappear.
Tidal forces at a black hole
are greater than any known
forces. Any object would be
stretched vertically and
horizontally squeezed. This
plus collisions among the
debris produce tremendous
frictional heating.
This frictional heating
produces radiation in
the form of x-rays
before the matter
reaches the event
horizon.
Light emitted by an
object motionless near
the event horizon would
be more and more
redshifted the closer it
moved to the event
horizon.
This is gravitational red
shift caused by the
huge gravitational
field and is a clear
prediction of Einstein’s
general theory of
relativity.
Photons lose energy to
escape a black holes gravity,
so they go to a lower
frequency and longer
wavelength. Photons at the
event horizon lose all their
energy and go to an infinite
wavelength.
A clock near the event
horizon would appear
to tick more slowly.
At the event horizon time
would appear to stand
still. This is called time
dilation.
An astronaut at
these points would
experience no
redshift or time
dilation in his frame
of reference.
How can black holes
be detected?
Stellar occultation?:
Probably not, due to the
bending of light due to
gravity.
The best method is
to look for binary
star systems which
show evidence of
black holes.
Properties of Cygnus X-1
• 1. A blue B-type supergiant in a binary
system.
• 2. Mass of the possible black hole 10 to 20 solar masses.
• 3. Gas flows from the supergiant to the
companion (black hole).
• 4. X-rays emitted by high temperature gas.
• 5. Rapid variations in X-ray emissions
suggest a short distance between the two
bodies.
The X-ray emitter is probably
an accretion disk.
There are perhaps six objects
that may turn out to be black
holes. Presently there is no
observational test that can
distinguish a black hole from
a neutron star.
The logic is:
Object X is compact and
massive. We don’t know
of anything that can be
that small and that
massive; therefore,
X is a black hole.
The above computer animated picture depicts how a very compact star
would look to a nearby observer. The star pictured is actually more
compact that any known except a black hole, so it is only hypothetical.
The observer is situated at the photon sphere, where photons can orbit in
a circle. To help the viewer better visualize the great distortions created by
gravity, a map of the Earth was projected onto the star, and a map of the
familiar night sky was projected above. From here one can either look
down and see several duplicate images of the entire surface of the star,
look up and see several duplicate images of the entire night sky, or look
along the photon sphere and see the back of one's own head.