by Michal Dorcak

Created on: September 28, 2008   Last Updated: February 25, 2010

Space is full of exotic objects. Many of them provide inspiration to artists or bring fear into hearts of most human beings. Black holes are indeed one of these objects. However, is black hole really an object?

What is a black hole?

If it is not an object, then what is this scary space “monster”? The answer is really simple. Black hole is a region in space. Specifically, it is the region which nothing, not even light, can escape. Objects that get too close to black hole cannot escape because of space-time deformation around it. This is caused by an extremely heavy and dense object (mass). There is an event horizon around the black hole. It is the point of no return. Even the light cannot escape from behind this point. This is why it looks like the surface of the black hole, even though it is not a real surface as we know it from our everyday life.

Spotting the black hole

Black hole itself is invisible. It is caused by light being unable to escape from behind the event horizon. So, if black holes do not reflect any light, how is it possible for us to see them?

Black holes can only be discovered by indirect observations. This mean that we observe gravitational effects they have on their surroundings. We can observe the accretion disk, which is composed of matter being sucked into the black hole. The potential energy is released due to friction within the accretion disk and heats up the gas falling in. It becomes so heated up that it releases vast amounts of radiation. However, not every accretion disk belongs to a black hole. Other extremely heavy objects (white dwarfs, neutron stars) can also have accretion disk.

Another possibility for spotting a black hole could be through phenomenon known as gravitational lensing. It means that light passing around a massive object is being deflected as if it was passing through an optic lens.

Classification

There are several types of black hole classified by their mass. These are supper-massive, intermediate-mass, stellar-mass and micro black holes.

Supper-massive black holes are thought to be in center of most galaxies. Our galaxy, the Milky Way is no difference. These gigantic black holes have from hundreds of thousands to billions masses of our Sun.

Intermediate-mass black holes are thousands of times more massive than our Sun. It is thought that they are created by collisions of stellar-mass black holes.

Stellar-mass black holes are created either by collapse of individual stars or by merger of binary

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