Gravity (part 2), Black Holes

So in my last blog, I talked about gravity and how it affects the Universe. This time, I will be talking about black holes.

What is a black hole? It is a region of space with a gravity so strong that nothing, not even light can escape. Black holes are formed by huge stars that collapse.

To understand how a black hole forms, let's look at the life cycle of a star. A star is formed when a large amount of gas, mostly hydrogen, starts to collapse in on  itself due to its gravitational attraction. As it contracts, the atoms of the gas collide with each other with increased frequency and at greater speeds. As a result, the gas heats up.

Eventually, the gas becomes so hot that when the hydrogen atoms collide they no longer bounce off each other, instead fuse together to form a colossal amount of heat, in the process forming helium. The heat released in this reaction, which is like a controlled hydrogen bomb explosion, is what makes the star shine. This additional heat also increases the pressure of the gas until it's sufficient to balance the gravitation attraction, and the gas stops contracting. The gravity tries to make the star smaller and denser but the heat generated inside the star expands the star. This keeps the star stable.

The star expands slowly overtime (about 13 billion years) until it becomes twice as big. As it gets bigger, it cools down by a few hundred degrees and turns red. A star like this is called a 'red giant'. When a star becomes a red giant, it's inching closer to its death. A star dies when it  has lost all its hydrogen fuel which kept it expanding. Small to medium sized stars like our Sun usually fade away when they have lost all their fuel leaving a small intact core (called a white dwarf). 


Bigger stars are much more dangerous. When a big star loses its fuel, gravity takes over and starts contracting the star. The reason why this does not happen with smaller stars is because smaller stars are not heavy enough to have a strong gravity. As the hydrogen fuel gets over, nothing can stop the star from contracting and the star keeps on getting smaller and smaller. As it shrinks, it gets denser and its gravity at the centre becomes stronger. Usually, when the star is 60 km in diameter, it's so dense that 1 tablespoon of the star would weigh about 2 cruise ships! 


When the star is 30 km in diameter, it becomes so dense that even light cannot escape its gravitational pull. This is when no light from the star goes far enough to be visible and the star is now a black hole. The star continues to get smaller and smaller until it reaches singularity. If a beam of light gets close enough, it will get pulled into the black hole. Anything that gets close enough is sucked into the black hole. A black hole the size of a neutron would weigh about a billion tones! That's really heavy. If one could hold a black hole, it would go straight through the hand and continue to go down through the floor and into the Earth until it reaches the centre.


Thanks for reading this, I will post another blog soon. Please comment