Black Holes

What are Black holes?

Introduction

As an amateur space explorer myself, one of the most interesting and exciting objects to me in deep space is black holes. There's no doubt that it has come of great interest to astrophysicists because of its mysterious or more convincingly, 'weird' behaviour. There are a lot of theories that revolve around black holes which might actually pave the way to new theories or laws that govern our universe. 

Here's an outline of basic facts regarding black holes including their formation:

• Black holes are created when a super-giant star burns out all of its fuel (supernova or death of a star), its core becomes so dense that it collapses under its own gravity. Clearly, not all burnt-out stars turn into a black hole. A star at least 20 times bigger than the sun can undergo this phenomenon.
• A black hole, as it's called, is completely black. This is because a black hole's gravity is so strong that not even light can escape its pull. That is why it's completely black.
• According to the classical theory of physics, gravity is directly proportional to mass, which means an object of more mass will have a stronger gravitational pull than an object of relatively smaller mass. The same principle can be applied in the case of black holes. Just like the moon orbits the earth and the earth orbits the sun, billions of stars, planets and other celestial objects orbit a black hole due to its unimaginably strong mass (gravity). 
• It is because of the above-mentioned property of black holes, galaxies are formed. So, black holes exist at the heart of every galaxy.

But because of its highly complex nature and weird behaviour, there's not much known about black holes. Yet, scientists are using whatever information they can collect to come up with new theories on the effects of black holes.

Some scientific terms regarding Black Holes

Singularity:

 The strong gravity inside a black hole that 

shrinks it to an unimaginably dense point is called a singularity. This is merely a point in the black hole where the space-time is bent into a funnel (cone) whose space-time curvature is infinite as well as the mass. Now, all the theories and models of our universe or space-time are based on the assumption that space-time is flat and because of this simple assumption, the singularity of a black hole is a point where all of these theories mentioned earlier, breaks down or no longer valid. This adds to the bizarre properties of black holes and thus gaining the attention of curious minds around the world.

Event horizon:

The event horizon or ' point of no return ' is basically a boundary on the surface of a black hole, beyond which nothing can escape. To be more specific, up to this boundary, an object can have an escape velocity (speed) not more than the speed of light. But once the object reaches the event horizon, the object should take up more than the speed of light to pull itself from the black hole's gravity. Since nothing can travel faster than light (according to Einstein), it's impossible to escape from the gravitational pull after reaching the event horizon.

However, things get more bizarre as an object approaches the event horizon. For a person viewing the above scenario, he or she won't see the object falling into the black hole even though it actually is. Instead, the viewer will notice that the object is moving (falling) much slower than the actual speed because on approaching the black hole, time slows down under the effect of gravity. The object will appear to be fainter (reddish) till it finally fades away. This is happening due to a phenomenon called red-shift.

Accretion disk:

The circular orbit-like part around a massive central body that contains the swirling matter and gases is called the accretion disk. In the case of black holes, the accretion disk emits X-rays and other cosmic waves due to the effect of different fundamental forces present during the ripping up of matter of stars and planets.