#### Transcript Black Holes

```A black hole is a region of space with
such a strong gravitational field that not
even light can escape.
For most of a star's
life, outward pressure
from nuclear activity
balances the inward
force of gravity.
When a sun runs out
of fuel, gravity
compresses the
material in the core
and it collapses under
its own weight.
If a star’s core is massive enough, nothing
can stop this collapse and the star is
compacted into a single point with zero
size and infinite density referred to as the
“singularity”.
The gravitational field of a typical black hole
is so strong that a person who weighs 100
pounds on Earth would weigh 30 billion tons
at the point just before he or she was
dragged into the black hole’s inescapable
gravity well.
For an object to escape a gravitational field, it
must reach escape velocity. On Earth, this equals
7 miles/second (11 km/sec). For a black hole, this
velocity reaches the speed of light (186,000
miles/second) at the point of no return.
But why is light, which has no mass, captured into
a black hole? Albert Einstein theorized that gravity
warps spacetime. Light, which ordinarily travels a
straight path, follows a curved path around a
gravitational field.
Physicists predict
that when gravity
distorts spacetime
to an extreme, the Normal effect of gravity on spacetime.
bottom falls out of
the curve, creating
the bottomless well
we call a black
hole.
Extreme curvature creating a black hole.
Black holes have
their own distinctive
anatomy, consisting
of:
• Singularity
• Event horizon
• Accretion disk
• Gas jets
The single point at the center of a black
hole where its entire mass is contained
is called the singularity.
The event horizon is the rim or boundary of
a black hole where escape velocity equals
the speed of light. Once inside the event
horizon, particles and light cannot escape.
At the event horizon,
dimensions as we
know them become
distorted. To an
outside observer, light
and time would seem
to stand still. An
object falling into a
black hole would
appear to slow and
stop at the event
horizon.
An accretion disk is a flat sheet of gas and
dust surrounding a black hole. This artist’s
conception suggests how an accretion disk
might appear.
Black holes may
have gas jets
caused by the
interaction of
gas particles
with strong,
rotating
magnetic fields.
These jets can
extend millions
of light years
through space.
Artist’s conception of a
black hole with gas jets
and an accretion disk.
Astronomers describe
three types of black
holes:
 Supermassive black
holes
 Stellar black holes
 Miniature black
holes
Artist’s conception
Supermassive
black holes can
have masses
equivalent to
billions of suns.
They are believed
to exist in the
centers of most
galaxies.
Orbiting stars may
be captured and
to the black hole.
Artist’s conception of a star being
drawn into a black hole.
The Hubble
Space
Telescope
has imaged a
number of
galaxies
believed to
have black
holes at their
centers.
Stellar black holes are produced by the
collapse of dying stars, and have masses
3 to 10 times that of our Sun.
Astrophysicists believe that miniature
black holes might have formed at
moment the universe was created, but
have no proof of their existence.
Miniature black holes have event
horizons as small as the width of an
atomic particle and contain as much
matter as Mt. Everest. Quantum theory
suggests that these black holes -- if they
exist -- may evaporate over time.
How do we know black holes exist?
A black hole itself is invisible because
no light can escape from it. It can be
found indirectly by observing its effect
on nearby stars and interstellar gases.
The image on the right is the signature of a
supermassive black hole in Galaxy M84, as seen
by Hubble’s Space Telescope Imaging
Spectrograph (STIS). The photo on the left shows
the slice of space that STIS was analyzing.
Astronomers also
measure x-rays from
the gas and dust of
surrounding stars
that are drawn into
orbit around the
black hole. The gas
becomes heavily
compressed and
friction among the
atoms causes x-rays
to be emitted.
Illustration comparing a
black hole and neutron star
in x-rays. The black hole
does not reflect light shown
by its dark center.
Using x-rays,
scientists can
measure the heat
and speed of
orbiting material,
and from this can
detect the presence
of a black hole. The
mass of the black
hole can be
determined by the
speed of the gas.
Chandra x-ray image of
galaxy cluster A2104.
```