What is a Black Hole?

Black holes are one of the strangest phenomena in the universe. Till a few decades ago they were a part of science fiction. However, there existence has now been proved. In fact, there’s a black hole (Sagittarius A*) at the centre of our own galaxy, the Milky way.

So, our Earth revolves around the Sun, and our Sun revolves around the black hole. So, in a way we all are revolving around a black hole only. Let that sink in.

In this article, we will discuss about the concept of black hole, its features, challenges in studying it, and what will happen if we get close to it or fall into it.

Table of Contents

• Types of Black Holes
• How are Black Holes formed?
• Can we see Black Holes?
• What is inside a Black Hole?
• What will happen if we fall into a Black Hole?
• How a Black Hole dies?

Types of Black Holes

Black holes may be of various sizes.

• Small Black Holes - Black holes having a mass of a star, or a few multiples of it, but having a size of only a few kilometres.

• Super Massive Black Holes – For examples, those found at the centre of most galaxies, including our own. They have the mass of millions of times that of a star, e.g. Sagittarius A* has the mass of 4 million Suns.

Largest known Black Hole

The largest black hole known so far is named S5 0014+81. It has 40 billion times the mass of our Sun. Its diameter is 47 times the distance of the Sun from Pluto.

How are Black Holes formed?

Black holes form after the death of a star. A star keeps on burning hydrogen by the process of nuclear fusion. Under tremendous heat and pressure of a star, the hydrogen atoms fuse into helium, and some mass converts into energy (in the form of radiation, i.e. gamma, beta, alpha particles). This further increases the heat and temperature.

This continuous production of energy balances the tremendous gravitational forces that want to crush the star.

This process goes on till there is some hydrogen left. Thereafter, the star starts burning helium and forming heavier elements – carbon, neon, oxygen, silicon, iron. However, once a star forms iron, it cannot burn it further. Once the amount of iron at the centre of the star reaches a critical mass, the fusion process stops and the star collapses under its own weight. Supernova happens - one of the biggest explosions in the universe. Gravity wins!

Tit Bits

• Iron is called the poison of a star.
• All elements heavier than iron are formed during Supernova.

Do all stars form Black Holes?

Not every star creates a black hole. Depending on the size of a star, it may form a neutron star, white dwarf star or a black hole.

• If the star is of small or medium size (like our Sun), then there will be no Supernova. The star will slowly burn out all fuel, and turn into much bigger red giant. Later on, it will end up as a white dwarf star.
• If the star is larger, it will die with a Supernova blast, and will form a neutron star.
• If the star is even more massive, then it will die with a Supernova blast and will form a black hole.

Can we see Black Holes?

No, we cannot see black holes. But why?

That’s because, black holes have such large gravity that not even light (photons) can escape from inside it. And if an object emits no light, then it cannot be seen. It’s just a dark sphere.

However, the outer layer of the black hole is called the event horizon. Photons and other particles may escape the gravity field of a black hole if they have not crossed the event horizon. Things are continuously being sucked by black holes, which causes heat and some particles are even produced from empty space near the event horizon. So, this outer layer of a black hole is dimly lit.

So, we cannot se a black hole, but we sure can see the area around event horizon.

What is inside a Black Hole?

No one knows what’s inside a black hole. Scientists call it singularity.

Singularity means all the mass is concentrated inside an infinitesimally small point of infinite density, with zero volume. However, scientists still do not know the nature of singularity. It’s something not defined, just as number/0 (a number divided by zero) is not defined.

But various scientists have come up with various theories and speculations regarding what’s inside a black hole.

• As per some experts, there’s a white hole on the opposite side of a black hole. The white hole may be in a separate universe, or may be creating a new universe by the process of big bang.
• According to some, it is just a point of singularity, wherein all laws of Physics fail.
• As per some, all the information that falls inside a black hole is not lost. It survives on its surface in the form of a hologram.

What will happen if we fall into a Black Hole?

Many of us are under the impression that black holes suck matter around them like a vacuum cleaner. But that’s not the case. They are just like other massive heavenly bodies that attract other masses towards them.

If we replace our Sun with a black hole having the same gravitational force, nothing will change as far as the orbits of the planets are concerned. They will keep on revolving around the black hole, just as they do around the Sun.

However, what if we throw something intentionally, or we ourselves move towards a black hole? Let’s take an example of an astronaut moving towards the black hole. And a scientist observing him moving towards the black hole from a space ship far away.

As the astronaut will get closer to a black hole, the time will slow down. It will completely stop when he will reach the event horizon. This happens because time tends to slow down near bodies of huge mass.

Variable Speed of Time

Time slows down:

• as gravity increases, or
• as speed increases. (When an object travels at the speed of light, mass becomes infinite and the time stops)

To the scientist observing from far away, the astronaut falling towards a black hole will seem to slow down. Moreover, such an observer will never witness that astronaut falling into the black hole. He will see that astronaut halted at the event horizon for an infinite time. That’s because from the perspective of the scientist, the time will stop at the event horizon.

However, the astronaut will not feel this slow down of time. He will just fall through the event horizon and then into the black hole. Moreover, from his perspective the rest of the universe will seem to travel fast ahead in time. So, in a way, while falling towards the black hole he will travel into the future.

But the astronaut will never reach the black hole alive anyways. The gravity near a black hole is so high, that the difference between the gravity near the foot and the head of the astronaut will be so high, that he will get stretched like a long spaghetti (called spaghettification or noodle effect).

How a Black Hole dies?

Though nobody knows for sure, but there’s a famous theory regarding it. We will write that down for you.

Space is not really empty. New particles keep on coming in existence in empty space. They remain in existence for a very short period of time, and then they destroy each other.

However, near the event horizon of a black hole, one of the particle may get caught by the black hole, and the other may escape as a real particle, as a radiation.

This is Hawking Radiation, named after a famous scientist Stephan Hawking, who died on 14th March 2018.

So, black holes keep on losing mass due to this Hawking radiation, i.e. they keep on evaporating. With time they die, but this process is very slow.