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.
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 neutron stars.
Micro black holes are thought to be created by highly energetic reaction when cosmic rays hit the atmosphere. They could also be created in particle accelerators. Anyway, there is no reason to worry about them sucking us in. They would evaporate before they could do that.
Yes, black holes really do evaporate. This has been discovered by Stephen Hawking. Black holes emit small amounts of thermal energy. This is known as Hawking radiation. However, black hole would have to be lighter than Moon to be able to evaporate. Also, the smaller the black hole the faster it evaporates. The problem is that evaporation releases energy (radiation). Energy released from evaporation of a small black hole is expected to be very strong. So, although the micro black hole would not swallow you, it could evaporate you instead (as would a nuclear bomb blowing up in your neighborhood).
Closest black hole
There is no need to worry about a black hole paying us a visit any time soon. The closest black hole to Earth that has been discovered is about 1600 light years away. It is situated on the way to the center of the Milky Way. Probability of black hole showing nearby and swallowing Earth just during our lifespan is almost nonexistent. Keep this in mind and do not live your life in fear of them. Rather look at them as something interesting and fascinating.