Black Holes -

In the previous blogs we discussed the death of various sized stars and concluded that the death of massive stars results in the birth of two of the most extreme bodies in our universe - neutron stars and black holes. We concluded with the discussion of neutron stars in the previous blog and today we will discuss the most extreme body in our universe - the black hole...

You may remember that the death of supermassive stars can result in the formation of either neutron stars or black holes. 

An illustration of a neutron star

When stars four to eight times as massive as the sun explode in a violent supernova, their outer layers can blow off in an often-spectacular display, leaving behind a small, dense core that continues to collapse. Gravity presses the material in on itself so tightly that protons and electrons combine to make neutrons forming a neutron star.

An artist's illustration of a supernova

What happens after the explosion depends on the mass of the neutron star. If it is less than three solar masses, (a solar mass is the mass of the sun) it remains as a neutron star. However, if it is more that three solar masses, it results in the formation of the strangest body in the universe - a black hole.

The first and only real image of a black hole

Credit: event horizon telescope collaboration

Black holes are the strangest and most mysterious objects in our universe. A black hole- quite contradictory to the common beliefs- doesn't suck things up like a vacuum cleaner. Just like the sun, it exerts a gravitational force on objects. This gravitational force is just very strong and appears to suck things up. The gravity of a black hole is so strong that if a star passes near the event horizon or inside the event horizon (explained later), the star will completely rip apart. The gravity of a black hole is so strong that even light - the fastest moving body in our universe, can't escape from a black hole. The only way you can escape a  black hole is by travelling faster than light which is not possible according to the special theory of relativity.

Structure of black hole:

Their outer boundary is called the event horizon. Anything passing through this boundary is lost forever as this is the point of no return. However, the real mystery of the black hole is the singularity. The singularity may an infinitely small, infinitely dense body, at the centre of a black hole, that is, all its mass is concentrated in a single point in space. Or it may be something completely different, scientists really don't know anything about it. It seems to make no sense at all.

The photon sphere is a result of the strong gravity of the black hole. Basically, the gravity even beyond the event horizon is so strong that the light around the black hole bends and goes circling around the black hole. To understand how bizarre this is, you can imagine this as light circling from the back of your head and curving to your eyes, meaning that you can see the back of your own head!

Supermassive black holes at the centre of galaxies create powerful jets of radiation and some dust particles that travel in speeds close to that of light. These are called relativistic jets.

Some materials like gas dust etc., that have come close to a black hole but haven't reached the event horizon, form a flattened band of spinning matter around the event horizon called the accretion disc. The innermost part of the accretion disc that isn't pulled into a black hole is called the innermost stable orbit.

          A very helpful infographic on the structure of black holes

Credit - ESO, ESA/Hubble

Now let's think about a commonly asked question- what would happen if we fell into a black hole? From the perspective of someone else watching you when you are dropped into a black hole, you would appear to go towards the event horizon slower and slower, until, it will appear as if you had frozen in time. Then you would cross the event horizon and disappear.

On the contrary, in your own perspective, you would see the universe move in fast forward, almost like seeing the future. 

After you cross the event horizon death is inevitable. In a black hole, the mass is so concentrated that gravity acts different on different parts of your body. In other words the gravity is so strong that even in tiny distances of few centimeters, the force of gravity acts with millions of times more force, your body completely rips apart and your cells are torn apart and all that's left of you is a hot stream of plasma only a few atoms wide.

The other possible situation is that as soon as you enter the black hole you hit a firewall of some kind and are instantly terminated. So you either die a quick death or a very quick death, none of which are very pleasant. How quick you die depends on the size of the black hole. 

You would die before you even reach a small black hole's event horizon and you would survive inside the event horizon of a supermassive black hole for a few seconds. Basically, the further you are from the singularity, the longer you'll survive.

There are still many many interesting and puzzling characteristics of black holes like the information paradox and the hawking radiation, but we'll talk about these in the next blog...