The amazing mystery of the black holes

Black Holes

Today, we will talk about one of the most mysterious bodies of our Universe – The Black Hole. Yes, The Black Hole! The place where a singularity comes into the picture, laws of physics break, time stops, the space-time warps and what not!

Black Hole - Just look at this magnificence!

But such is the binding by the word limit and the need to keep it short and simple, we cannot discuss as much as we would wish to. Thus, we would talk very briefly about black holes in this article, mainly their formation and some of their properties. Topics related to it, such as its parts, nature of particles around it, the fate of us if we fall into one, its death, etc. would come following. So, let’s start!

To understand a black hole, to define it, we have to look at its origin.

As we know, stars keep on burning and releasing enormous amounts of energy due to nuclear fusion which takes place in its core. Hydrogen fuses to form helium giving a huge amount of energy. The energy in the form of radiation pushes against gravity. Thus, preventing the massive crust from collapsing into the core. This maintains a delicate balance between the two forces.

Balancing the two forces

But what happens when all the fuel is consumed?

For a star that has about one half the mass of the sun is too small and too cool to fuse helium to carbon. Thus, it ends up becoming a white dwarf made of helium. Stars between one half to four times the mass of the sun are massive and hot enough to fuse carbon to oxygen. Carbon and oxygen fuse more or less at the same time and a white dwarf made of carbon and oxygen is obtained. Due to a large amount of heat and pressure, we may have a white dwarf made of diamonds! Stars with masses greater than four times the mass of the sun are massive and hot enough to fuse oxygen to silicon. Stars that have earned the title of “supergiant” are so massive and hot that they begin fusing silicon into a solid core of iron.

White Dwarf

Lucy - A diamond star 4000 Km across

Fusing silicon to iron takes more energy than it gives off. Thus, the balance between radiation energy and gravity is broken. The star begins to shrink under its own gravity. If the iron core builds up to 1.4 times the mass of the sun, it cannot survive the pressure and collapse. In some cases, matter slams into the core resulting in shock waves that trace back and blow the star apart, ending it in a supernova. A supernova releases incredible amounts of energy and is responsible for the formation of heavier elements in our Universe.

Supernova

When a large star implodes, its weight is enough to squash atoms in its core, down to its nuclei. This results in an ultra-dense neutron star. We can imagine the density of neutron stars to be that of a mountain being crushed into the size of a marble! Neutron stars due to their ultra-density can withstand enormous amounts of pressure. But if enough matter falls upon it, above a critical threshold, they will collapse into absolute nothingness! And when that happens, a black hole is born. Mass is packed into a single point known as a singularity. (Singularity is a point with infinite density and our general understanding of physics fail here.)

Neutron Star

A decent-sized black hole is called a stellar-mass black hole with a diameter of an asteroid. There are also super-massive black holes with mass millions to billions of times the mass of the sun. We have one at the centre of every galaxy, including our Milky Way! It offers a great insight into the formation of these galaxies as they must have been interdependent.

An artist's depiction of the black hole at the centre of the Milky Way Galaxy

A common idea about a black hole is that it is made up of matter very compact. But this is wrong! The matter gets completely destroyed. It no longer exists. Yet it leaves behind one of its most powerful legacies – The Gravity!

According to Albert Einstein, gravity is not the force that attracts two objects but it is the warping of space-time around a massive object. A black hole is a deep puncture in space-time. The energy of matter is converted into energy of warped space-time.

Gravity as described by General Theory of Relativity

One more misconception that is widely held is about the size of a black hole. A black hole has no actual size. It is a singularity. The black boundary that you often see in the pictures, represents the part of the black hole, where the gravitational pull is so strong even light cannot escape (And as light is necessary for an object to be visible the region appears black). The boundary of the black part is called the event horizon. But, the radius of the event horizon does depend upon the mass of the black hole. To give it a name, this radius is the Schwarzschild radius. It can be calculated for any mass as

Radius = 2GM/c² 

               Where    G is the Gravitational constant.

                              M the mass

               And,       c is the speed of light.

Thus, r = 1.48*10^-30 M meter.

Or, we can say that if we crush a body into its Schwarzschild radius, we will obtain a black hole!

Event Horizon, Singularity and the Schwarzschild Radius

Well, that was all for the day. I hope you liked the article. Don't forget to calculate your Schwarzschild radius!

Bis später!