Black holes are objects with extremely strong gravitational fields that do not allow anything, including light, to escape once it passes the event horizon. They are formed when very massive stars collapse at the end of their life cycles. Black holes are small but incredibly dense, with all their mass concentrated at a central singularity. If material such as gas from a nearby star falls into a black hole, it becomes heated and glows, making the black hole visible with telescopes.

2. Black holes are objects
of extreme density,
with such strong gravitational attraction,
that even light cannot escape from their grasp
if it comes near enough.
Definition

3. . Albert Einstein first predicted
black holes in 1916.
The term ‘black hole’was coined
in 1969 by the American scientist John
Wheeler.

4. What does a black hole look
like?
As gas falls into a black hole (perhaps
coming from a nearby star), it will heat up
and glow, becoming visible.
Typically, not only visible light, but also
more energetic photons like X-rays will be
emitted by the gas.
What we would expect to see (if our
telescopes could "zoom-in" enough) would
be a glowing rotating disk of material, with
the black hole down a the center of the disk.

5. When a star burns through the last of its fuel, it may
find itself collapsing.
For smaller stars,
the new core will be a neutron star or a white dwarf.
But when a larger star collapses, it continues to fall in
on itself to create a stellar black hole.
How a star becomes a black hole?

6. Structure

7. The event horizon of a
black hole is the
boundary around the
mouth of the black hole
where light loses its
ability to escape.
The inner region of a black
hole, where its mass lies, is
known as its singularity, the
single point in space-time
where the mass of the black
hole is concentrated.

8. Cygnus X-
1

9. How
the
black
holes
tick?
Black holes
do not pull
objects like a
vacuum.
Instead,
objects fall
into them.

10. Light is stationary.
Light is pulled in.
Light escapes.
Inside the event horizon, space is
being pulled faster than the speed
of light.
Black holes are incredibly
massive, but cover only a small
region.
Because of the relationship
between mass and gravity, this
means they have an extremely
powerful gravitational force.

11. One could
well say of
the event
horizon
what the
poet Dante
said of the
entrance
to Hell:
objects can fall through the event horizon into the black hole, but nothing
can ever get out of the black hole through the event horizon.
The event horizon, the boundary of the region
of space-time from which it is not possible to escape,
acts rather like a one-way membrane around the black hole:

12. Chandrasekhar limit
Subrahmanyan Chandrasekhar worked out how big a star
could be and still support itself against its own gravity after
it had used up all its fuel.
Chandrasekhar limit = the maximum possible mass of a
stable cold star, above which it must collapse into a black
hole.
Chandrasekhar limit = about one and a half times the mass

13. Stephen Hawking’s
example with the
astronaut
An intrepid astronaut on the
surface of the collapsing star, collapsing
inward with it, sent a signal every second,
according to his watch, to his spaceship
orbiting about the star. As 11:00
approached his companions watching
from the spaceship would find the
intervals between successive signals
from the astronaut getting longer and
longer. They would have to wait only very
slightly more than a second between the
astronaut’s 10:59:58 signal and the one
that he sent when his watch read
10:59:59, but they would have to wait
forever for the 11:00 signal.

14. Stephen Hawking’s
example with the
astronaut
The singularity
will be always in
the astronaut’s
future, but never
in his past.

15. Stephen Hawking is one of the scientists who have worked on the
theories about the black holes. In ‘Brief history of time’ there is an
interesting , but impossible idea (for near future) about the
usage of the black hole’s power:
Because of the radiation from the black holes in the form of X
rays and gamma rays, black hole could run ten large power
stations, if only we could harness its power. We can not have one
of these black holes on the surface of the earth, and the only
place to put such a black hole, in which one might use the energy
that it emitted, would be in orbit around the earth – that is the
impossible step for now.
But, with the development of science and technology, in far future,
people might find out much more about our universe, including
the black holes, and would make them useful for their living.