# Black Holes

## Overview :-

For many years there have been several debates on the existence of black holes, but now we know that black holes do exist. Black holes are the strangest and the most powerful things in the universe. They are strong enough to rip apart large stars into atom sized pieces. They are so strange that they might delete the universe. In this module we will be learning about these strange things.

## What are Black Holes?

A black hole is a place in space where gravity pulls so much that even light cannot get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying. A black hole takes up zero space, but does have mass — actually, most of the mass that used to be a star. And black holes get bigger as they consume matter near them.

The concept of a black hole can be understood by thinking about how fast something needs to move to escape the gravity of another object – this is called the escape velocity. Formally, escape velocity is the speed object must attain to break free of the gravitational attraction of another body.

There are two things that affect the escape velocity – the mass of object and the distance to the center of that object. For example, a rocket must accelerate to 11.2 km/s in order to escape Earth's gravity. If, instead, that rocket was on a planet with the same mass as Earth but half the diameter, the escape velocity would be 15.8 km/s. Even though the mass is the same, the escape velocity is greater, because the object is smaller and denser. If we squished the Earth's mass into a sphere with a radius of 9 mm, the escape velocity would be the speed of light. But the speed of light is the highest speed limit, so it would be impossible to escape that tiny sphere, if anyone got close enough. You could relate this to a situation. Imagine you are trying to swim in the opposite direction of a waterfall, so unless you don’t swim faster than the speed of the water you will definitely fall down the waterfall.

The radius at which a mass has an escape velocity equal to the speed of light is called the Schwarzschild radius. Any object that is smaller than its Schwarzschild radius is a black hole – in other words, anything with an escape velocity greater than the speed of light is a black hole. To make a black hole a star the mass of our sun would need to be squeezed into a volume with a radius of about 3 km.

## If there is no light, then how can we see a Black Hole?

Astronomers don't exactly see black holes directly. Instead, astronomers observe the presence of a black hole by its effect on its surroundings.

Imagine you arrive home one night to find there is a mess in the kitchen. You know that it was clean when you left, but now there are dirty dishes in the sink and crumbs scattered. From this, you know someone used the kitchen while you were out – in fact, you can even understand what they made by the crumbs. You might even be able to identify who from your household has eaten it by looking at the pieces of the crumbs. You never saw that person in the kitchen, but their effect on the kitchen was evident.

Studying black holes relies on indirect detection. Astronomers cannot observe black holes directly, but see behaviors in other objects that can only be explained by the presence of a very large and dense object nearby. The effects can include materials getting pulled into the black hole, or stars orbiting a massive and unseen object.

## How are Black Holes formed?

Massive Stars are collections of hydrogen atoms that due to nuclear fission convert into Helium atoms and it releases a lot of energy. This is how stars like the sun produce light. This energy in the form of radiation pushes against the gravity and maintains a balance between these two. So, we can say that until there is fusion in the core there will be a balance and the star will stay stable.

But for stars that are much more massive than the sun, due to the immense heat and pressure heavier elements like iron fuse. But the fusion of iron doesn’t release any energy and it builds at the centre of the star. Due to this the balance between radiation and gravity breaks.

This all results in the star to collapse and it implode, taking more mass at its centre. Now the star dies in a supernova explosion. This produces a neutron star or if the star is huge enough the whole mass collapses into a black hole.

## Structure of a Black Hole :-

There are two basic parts to a black hole: the singularity and the event horizon.

The event horizon is the point of no return around the black hole. It is the surface surrounding the black hole that marks where the escape velocity is equal to the speed of light. Its radius is the Schwarzschild radius.

Once matter is inside the event horizon, that matter will fall to the center. With such strong gravity, the matter compresses to just a point – a tiny volume with a huge density. That point is called the singularity. It is so small, so it has infinite density. But still there isn’t adequate information about the Singularity.