Black holes are some of the strangest and most fascinating objects in existence. They
are extremely dense with such immense gravitational attraction that not even light, which is the
fastest entity in our universe, can escape their grasp. This means we cannot really see
a black hole since no light can be reflected off its surface. So, how did we first discover
them? And how do these mysterious objects even form? Let’s explore some of these mysteries
behind black holes!
Albert Einstein was the first to predict the existence of black holes in 1916, with his
general theory of relativity. The term "black hole" was coined many years later in 1967
by American astronomer John Wheeler. After decades of being known only as theoretical
objects, the first physical black hole ever discovered was spotted in 1971 by astronomers
who were studying the constellation Cygnus. But we cannot directly observe black holes
since they are invisible due to no light being reflected off their surfaces. So, scientists
study them by observing their effect on nearby matter. For example, black holes emit radiation
when gas and dust are drawn towards them forming accretion disks. Or quasars which are bright
jets of particles blasting off of supermassive black holes.
A Blackhole consists of a boundary known as the event horizon, past which no particles
or even electromagnetic radiation such as light can escape, and the singularity which
is the single point in space-time where all the mass of the black hole is concentrated.
Once something crosses the event horizon, it collapses into the singularity which is
an infinitely small and infinitely dense point where all our known laws of physics break
down. Even space-time as we know it no longer exists at the singularity! Because of this,
scientists do not know what happens at the singularity of a black hole. The problem is
analogous to dividing by 0 in mathematics. We simply do not have an answer for either
one yet!
There are four different types of black holes, stellar, supermassive, intermediate, and miniature.
Stellar, and supermassive black holes are the most common among these.
Stellar black holes are formed when very large stars, about 8 to 10 times the mass of our
sun, burn through the last of their fuel. They start to fuse heavier and heavier elements
until they reach iron when the core collapses and the star implodes adding even more mass
to its core. This results in a supernova explosion resulting in the death of a star and the birth
of a black hole. Black holes formed by the collapse of individual stars are relatively
small but incredibly dense. Scientists estimate that our Milky Way galaxy alone may contain
a few hundred million stellar black holes!
While Stellar black holes populate the universe, supermassive blackholes dominate. They can
be millions or even billions of times as massive as our sun with relatively bigger diameters
than Stellar black holes. These supermassive black holes are thought to lie at the center
of all galaxies including the Milky Way. The Supermassive black hole at the center of our
galaxy is known as Sagittarius A. It has a mass of about 4 million times our sun and
has a diameter of roughly the distance between the Earth and our Sun. Now, scientists are
not certain how these supermassive black holes form, but they do have a few theories. Some
believe they may be the result of hundreds or thousands of tiny black holes that merge
while others look to dark matter for answers. Regardless, no one knows for certain what
the actual case may be.
Now it was once thought that black holes come in only these two sizes. But in 2014, astronomers
found what appeared to be a midsize or an intermediate-mass black hole in the arm of
a spiral galaxy. Further research found that black holes with about 10s to 100s of thousand
times the mass of our sun could lie at the heart of dwarf galaxies. It is theorized that
such bodies could form when stars in a cluster collide in a chain reaction eventually falling
together in the center of a galaxy and create a supermassive black hole.
Now, these are not the only ways of making black holes. While studying Einstein’s field
equations in 1916, a German physicist named Karl Schwarzschild proved that any mass could
become a black hole if it was compressed to a certain degree.
He called it the Schwarzschild radius and defined it as the radius below which the gravitational
attraction between the particles of a body must cause it to undergo irreversible gravitational
collapse.
The Schwarzschild radius (Rg) of an object of mass M is given by the following formula,
in which G is the universal gravitational constant and c is the speed of light: Rg = 2GM/c^2.
So, even we humans can become blackholes if we were compressed tightly enough. But for
a mass as small as that, the Schwarzschild radius is of the order of 10^-25 m which is
much smaller than even the nucleus of an atom!
Now, black holes may sound intimidating and everlasting, but eventually, even they will
die through the process called Hawking radiation. This thermal radiation arises from the steady
conversion of quantum vacuum fluctuations into pairs of particles, one of which escapes
the event horizon while the other collapses into the singularity. This causes black holes
to lose energy over time until they evaporate completely out of existence!
And that brings us to the end of this video! If you enjoyed it, do leave a like, share,
and subscribe to our channel for more content ahead! Until then, we’ll see you in the
next one!