Black holes are a tantalising lot. Celestial objects that defy our wildest imaginations of size and power. They are so hard to detect and view, that their existence was confirmed theoretically almost a century before we could lay our eyes on one.
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What are black holes?
Simply put, a black hole is a point in space with gravity so powerful that the fastest travelling thing in our Universe cannot escape. So what is the fastest travelling thing in space? Light is the fastest travelling thing in space. It travels at a mind-boggling 300,000 kilometres per second. Effectively speaking there is nothing in our known Universe that can get close to a black hole and still escape from getting sucked in.
Why are black holes called so?
This unique ability of black holes also gives birth to their name. Since light cannot escape from it, we will never be able to see a black hole. We see objects only when light bounces off them.
Also, we are yet to understand what happens to objects that get swallowed up by a black hole. Hence the name ‘black hole’ seems apt to describe them.
How are they formed?
We know that objects that are bigger and heavier in space tend to have more powerful gravitational pulls. Earth has a mass 81 times more than the moon. Its gravity is 6 times more powerful than the moon.
Jupiter, the largest among the planets in the Solar System has a mass 319 times that of the Earth. However, on account of its gaseous nature, the gravity is 2.5 times that of the Earth.
Our Sun is 10 times larger than Jupiter and 1000 times more in mass. Its gravity too is 1000 times more than that of Jupiter.
Black holes are extremely massive. Their mass is concentrated in a tiny amount of space. This happens when stars of a certain size die. As they die, their entire mass collapses into a tiny fraction of space as was occupied by the erstwhile star. This massive concentration of matter into a tiny space creates a black hole.
Why do black holes have such strong gravity?
Black holes are associated with extremely strong gravitational fields due to the small space in which matter gets concentrated.
To explain with an example, our Sun will turn into a black hole if all the matter in it gets compressed into a ball of size just half a mile in diameter. Our Sun currently has a diameter of 865,370 miles.
The concept of density is worth explaining here. Density is Mass over Volume
As matter gets squeezed into smaller and smaller regions of space, the density starts increasing. The greater the matter squeezed into a fixed space, the greater the density. Solids are denser than most liquids. Liquids are denser than gases.
While a spoonful of matter from our Sun will weigh about 740 grams, a spoonful of matter from a medium-sized black hole will weigh about 100,000,000,000,000 grams. Hence a black hole is also described as a region of infinite density.
How do black holes differ from their stars if the mass is the same?
A black hole and the star that created it has the same mass. If masses are the same, how does gravity differ? They do not differ. The gravitational pull is the same. Then what differs?
To understand this, one has to understand the concept of space-time. It was Albert Einstein who gave us the concept of space-time. He states that the space we see around us is actually akin to a fabric stretched apart. If any heavy ball is placed on this fabric, there is a depression created around this ball in the fabric. It is this depression called a ‘gravity well‘ that causes the fabric to curve and give rise to gravity.
The greater the depression, the greater the gravitational effect of the celestial body that is represented by our ball.
Imagine, letting loose a smaller ball around the bigger ball on the fabric. The smaller ball will start going around the bigger ball and after a few revolutions, eventually, join the bigger ball.
This is exactly what is happening at a cosmic scale with the Earth and the other planet revolving around the Sun. They are trapped in the distortion of the ‘space-time’ fabric caused due to the mass of our Sun and hence are eventually doomed to fall into the Sun.
Coming back to black holes, the depression in the fabric of space-time caused by these extremely dense objects is so great that it creates an almost endless gravity well. Once you start slipping down this well, you are doomed. There is no clawing back.
Are there different types of black holes?
Black holes are classified by NASA into three different types
- Primordial Black Holes
- Stellar Black Holes
- Super Massive Black Holes
Type 1 – Primordial Black Holes
These black holes formed during the early formative years of our Universe, roughly the first 2 billion years after the Big Bang. These black holes could have been the size of atoms but with the mass of a mountain.
The theory is that after the Big Bang, the matter was not uniformly distributed across the Universe. Clumps of matter that were closer coalesced to form the first primordial black holes. A new theory suggests that it could be these could have been the harbingers of the supermassive black holes we see at the centre of every galaxy
Type 2 – Stellar Black Holes
Black holes that form as a result of the death of a star are termed stellar black holes. Typically stars that are massively larger than our Sun end up becoming black holes. Among observed instances, stars that had a mass of 17 times or more than the Sun have turned into black holes at the end of their lives.
Type 3 – SuperMassive Black holes
These are the black holes that reside at the centre of every galaxy. The one at the centre of our Milky Way galaxy is called the Saggitarius A. These SMBHs are responsible for holding the galaxy together.
An SMBH is so big that it is of the order of millions to billions of times the mass of our Sun.
