Black holes are massive objects scattered across the universe, possessing such strong gravitational fields that not even electromagnetic radiation can escape the pull of the black  hole. Yet, there is a lot to be known about these objects, which have garnered the interest of physicists, writers and artists alike.

Let us now witness one of the most luminous and energetic events in our universe: the death of a star.

The formation of a black hole: stellar black hole

Every star in this universe undergoes a stellar evolution process, though the star’s final fate is determined by the mass of its remnant. Once the main sequence star is extinguished of hydrogen in its inner core, the hydrostatic equilibrium of the star is tilted in favour of the gravitational force originating from the star’s centre of mass. However, the rapid contraction of the core leads to a rise in the temperature of the core of the star, and the temperature eventually rises to the point where hydrogen fusion commences in the outer layers of the core. The fusion leads to an increase in pressure and the outer layers of the star expand greatly, as the pressure arising from nuclear fusion counters the inward gravitational contraction of the core. Depending on the mass of the star, various fusion processes occur in the core, producing heavier elements from fusion, beginning from fusion of helium in the core.. This process halts at the production of iron, which occurs in the heaviest of stars. Eventually, the helium in the shell of the core is extinguished, and this marks the end of the star’s red giant stage. With all fusion processes halted in the core, the radiation pressure of the outer layers of the stars are blown away in a planetary nebula. The core that is left over is known as the stellar remnant. If the stellar remnant is below 1.4 solar masses (Chandrashekhar Limit), the core forms a white dwarf due to electron degeneracy pressure. The Pauli Exclusion principle does not allow two electrons to possess the same energy level in one particular space. This forces electrons to occupy higher energy levels, and by doing so, the movement of these high energy electrons creates a pressure which counters the gravitational pull of the remnant.

In the case of larger stellar remnants, the star’s outer layers are blown off in a supernova. In the red supergiant stage, as fusion reactions lead to the production of heavy elements, the core temperature grows hotter, to the point where photons gain enough energy to split nuclei. This leads to free protons, neutrons and electrons in the core of the star. Due to the extreme conditions in the core, electrons and protons combine to form neutrons and neutrinos. As the neutrinos escape the core, the neutrons are packed together in a very small space, and this leads to the rise of neutron degeneracy pressure, which functions on the same principle as electron degeneracy pressure. However, the core still contracts due to the gravitational pull, but due to the countering neutron degeneracy pressure, the core rebounds to regain equilibrium. This rebound creates a massive shockwave, travelling through the outer layers of the stars. This blows the outer layers apart in a violent supernova explosion. This supernova is far more energetic than a planetary nebula. If the remnant from a supernova is within 2-3 solar masses, then a neutron star forms. However, beyond 2-3 solar masses, even neutron degeneracy pressure cannot counter the immense gravitational force, and the stellar remnant collapses to form a stellar black hole. Larger black holes, such as intermediate mass, massive or supermassive blackholes, do not form through the process of stellar evolution. The origins of these blackholes are still uncertain.

We have witnessed the death of a star and birth of a black hole. Let us now see if theoretically, we can create a black hole in our backyard.

Schwarzschild Radius:

Theoretically, any object can collapse into a black hole if its mass is compressed beyond its Schwarzschild radius. One of the unique properties of a black hole is that light is also unable to escape the gravitational pull of the black hole. Therefore, it can be concluded from this fact that to exit beyond the event horizon of the black hole, one would have to accelerate to the speed of light. Therefore, if one is to escape the black hole, the minimum escape velocity is equal to the speed of light, c.

Consider the equation for escape velocity:

$ V_{esc}=√\frac{2GM}{R} $

Where M is the mass of the object, and R is its radius.

Now consider the fact that the escape velocity of a black hole is c, the speed of light, so the left hand side of the equation would be replaced with c. Thus, the new equation is:

$ c=√\frac{2GM}{R} $

Squaring both the sides yields the following equation:

$  c^2=\frac{2GM}{R}  $

Solving for R yields this equation:

$ R=\frac{2GM}{c^2} $

This is the equation for the Schwarzschild radius. However, in addition to calculating the radius an object would have to be compressed to in order to form a black hole, the Schwarzschild Radius also provides useful information on the event horizon of a black hole. The event horizon of a black hole is the point near the black hole beyond which the escape velocity is c. For a non-rotating or Schwarzschild black hole, the Schwarzschild radius is also the radius of the event horizon of the black hole.

Black holes aren't of just one kind though. Much like stars, there are different kinds of black holes.

Schwarzschild vs. Kerr black hole

While black holes cannot be observed as other interstellar objects are observed, they do have certain properties which can be measured, such as mass, charge and angular momentum. On the basis of angular momentum, black holes can be divided into two types: Kerr black holes and Schwarzschild black holes. Rotating black holes, possessing angular momentum are Kerr black holes whilst non-rotating black holes, Schwarzschild black holes, possess no angular momentum. Additionally, Kerr black holes also possess an element known as the ergosphere. This is the point before the event horizon of the black hole where the effects of the black holes gravitational pull is felt, but the escape velocity isn’t yet c.

So far, we've seen the birth of a black hole, explored the possibility of creating your own black hole, and the different kinds of black holes. With that, this article comes to an end, much like the ill-fated star in the first paragraph did.