Before we attempt to understand about a black hole, first question to be addressed is, why to care about a black hole when the closest to the Earth is almost 3000 light years away. According to Newton’s universal law of gravity, given by F = G*m1*m2/(r*r), two bodies with mass exerts a gravitational force on the other. Heavier the mass, larger the force.
This was accepted as the universal law until Einstein published his theory of General Relativity. While Newton believed the reason Earth revolved around Sun was gravity, Einstein’s theory suggested, it is the bend in the space-time fabric which holds Earth to revolve around the sun, this also applies to other bodies in the universe. While this doesn’t disprove Newton was wrong about gravity, Einstein’s theory upholds gravity suggesting cause as the bend in space-time fabric.
What is space-time fabric? The common notion about space is, “Space is empty”. While this still holds true, until we know what is out there. It is believed space is comprised of particles, creating the fabric. Some theories which try to explain this is, String theory, Higgs Boson — The famous God particle and, etc.
While we can go deeper to understand more about the space-time fabric, the easiest way of imagination is, if we shrink Sun to the size of a football of 1 ft diameter, planets to the size of a tennis ball (3 inch), get a 1-meter * 1-meter cloth, place the football at the center of the cloth, distribute the tennis balls, Can you imagine the bend in the fabric, and, how the tennis balls are moving closer to the football? The bend here is due to the heavier mass of the football, and, the tennis balls move closer to the football, and not the other way.
Unlike the football scenario, planets don’t move in the direction of gravity to collide with stars. Example, Earth has its own velocity attained at the time of creation, and, acting perpendicular to the force of space-time fabric. As per Newton’s law of motion, “An object continues to move with a constant velocity, unless there is an external impeding force”. With no external colliding objects on the earth’s path, it smoothly rotates on its axis, giving rise to, day, &, night, influenced by the space-time bend, revolves around sun creating seasons such as, summer, winter, and, etc.
Now, since we can imagine space-time fabric, let’s learn a little about how stars age. Primary source of energy in stars is nuclear fusion. Example, most of the stars like our sun are made of Hydrogen, which undergoes nuclear fusion to form helium and releases energy, given by the equation e = mc2.
In simplest terms, Hydrogen + Hydrogen => Helium + Energy.
With time, Helium atoms gets accumulated at the core, and, hydrogen gets depleted. When not enough hydrogen is left for fusion, (Sun is expected to reach this stage in another 5 billion years), the hot and dense helium core explodes to a Supernova. This explosion creates heavier iron atoms.
Helium core + not enough hydrogen => Supernova + denser core of Iron.
When the mass of the star is more than 2–3 times Sun’s mass, the iron core collapses on to itself. The core collapse here refers to a tug of war situation. When the star has enough fuel to burn, the released energy creates pressure away from the star, which balances the force of gravity created by the core. When this is happening, the size of the star is constant. Imagine this to a flying aircraft. As long as the aircraft has enough fuel to burn, it can fly at a constant rate. When the fuel is exhausted, and, the pilot decides not to land, the aircraft gets dragged by the earth’s gravity due to the lack of forwarding thrust and crashes on the ground. Why doesn’t the core of the earth collapses on to itself? Because the earth is not bigger enough for this to happen.
Supernova => mass greater than Chandrashekar limit (i.e., 1.4 * Sun mass) => neutron star => further collapse => Mass greater than 2–3 Sun mass => Blackhole
Sun => Supernova => White dwarf
When the core collapses onto itself, all the mass the star had gets contracted to a much smaller size. Example, if the sun was 8 times heavier that what it currently is, after the collapse, it will shrink to an object of size with diameter of just 15 miles. This creates a massive abyss in the space-time fabric, so powerful that even light cannot escape from its gravity.
In general, black holes, first predicted by Einstein in 1916, gives better understanding of the space as a fabric, and, time, as a variable quantity, and, a dimension influenced by the bend in space.