THE BLACK MYSTERY CONTINUES…..
ASANSOL ENGINEERING COLLEGE
1ST OF ALL LET US ANSWER SOME
FREQUENTLY ASKED QUESTIONS AND
BASIC QUERIES ABOUT THIS MYSTERY.
1. What is a BLACK HOLE actually?
A black hole is a region in space where gravity pulls
so much that even light can not get out. The gravity
is so strong because matter has been squeezed into
a tiny space. This can happen when a star is dying.
Because no light can get out, people can't see black
holes. They are invisible. Space telescopes with
special tools can help find black holes. The special
tools can see how stars that are very close to black
holes act differently than other stars.
2.HOW BIG ARE BLACK HOLES?
Black holes can be big or small. Scientists think the
smallest black holes are as small as just one atom.
These black holes are very tiny but have the mass of
a large mountain.
Stellar black holes= Its mass can be up to 20 times
more than the mass of the sun.
Super massive black holes=The largest black holes
have masses that are more than 1 million suns
together. The super massive black hole at the centre
of the Milky Way galaxy is called Sagittarius A. It has
a mass equal to about 4 million suns and would fit
inside a very large ball that could hold a few million
How Do Black Holes Form?
Scientists think the smallest black holes formed
when the universe began.
Stellar black holes are made when the center of a
very big star falls in upon itself, or collapses.
When this happens, it causes a supernova. A
supernova is an exploding star that blasts part of
the star into space.
Scientists think super massive black holes were
made at the same time as the galaxy they are in.
Formation of a black hole
Stage wise formation of a black hole
If Black Holes Are "Black," How Do
Scientists Know They Are There?
A black hole can not be seen because strong
gravity pulls all of the light into the middle of the
black hole. But scientists can see how the strong
gravity affects the stars and gas around the black
hole. Scientists can study stars to find out if they
are flying around, or orbiting, a black hole.
When a black hole and a star are close
together, high-energy light is made. This kind of
light can not be seen with human eyes. Scientists
use satellites and telescopes in space to see the
Why light can not escape a
We know on earth that if anything is to escape
the earth, it should be projected at a speed of
11.8 km/sec. This means that the escape velocity
earth is 11.8 km/sec.
But in the case of black hole the scenario is
entirely different. Due to its huge gravitational
pull the escape velocity from a black hole is even
greater than the SPEED OF LIGHT which means
greater than 3*10^8 m/sec. Hence we can
conclude that even light cannot escape from it &
thus a black hole appears BLACK.
Why do two black holes come closer
More than half of the stars in a galaxy are
double stars and there is every possibility that
both the stars in a binary would become black
According to classical electro-dynamics,
accelerated electric charges radiate electro
magnetic waves and thereby lose energy
similar to gravitational bodies.
Thus black holes have a great tendency to
merge with each other.
Stages in black hole
The final merger of two black holes can be
divided into 3 stages:
1. Inspiral……2. Merger….3. ringdown
Inspiral stage= in this stage, the holes
are relatively far apart and can be considered
as point masses. As these go on radiating
gravitational waves, their orbits go on
contracting and, in accordance with Kepler’s
3rd law of planetory motion.
When the two holes come very close and only
a few final orbits are left, the holes leave their
orbits and plunge together in a rapidly
The event horizons of the two merge to form
a single highly distorted black hole.
The distorted single black hole that has
resulted from the merger radiates away all its
distortions in the form of gravitational waves
and becomes a smooth, symmetrical,
rotating black hole.
The process is similar to what happens when
a bell is struck by a hammer, the bell radiates
all its distortions as sound waves and
ringsdown and so the name ringdown.
Types of black holes…
1. Stellar black holes= these have masses about 3
to 30 solar masses and which are formed due to
exhaustion of thermal nuclear fuels.
2. Intermediate masses= these have masses
equal to 10^2 to 10^3 solar masses which form
from merger of small black holes in the centers
of dense stellar clusters.
3. Supermassive black holes= these have masses
equal to 10^4 to 10^10 solar masses which are
found in the center of most galaxies.
Could a Black Hole Destroy Earth?
Black holes do not go around in space eating stars, moons
and planets. Earth will not fall into a black hole because no
black hole is close enough to the solar system for Earth to
Even if a black hole the same mass as the sun were to take
the place of the sun, Earth still would not fall in. The black
hole would have the same gravity as the sun. Earth and the
other planets would orbit the black hole as they orbit the
The sun will never turn into a black hole. The sun is not a big
enough star to make a black hole.