Often considered as cosmic vacuum cleaners, black holes are much more mysterious as well as interesting, captivating minds of both scientists as well as common people. In fact, they are the evolutionary end point of massive stars.
To gain more insight into what they actually are, we need to look into some more important points about them. When stars at least 10 to 15 times larger than our own Sun undergo supernova a fair amount of stellar remnant is left. Such stellar remnants have no outward forces to oppose gravitational forces and thus, this remnant will collapse in on themselves. This process of collapsing continues until stage of zero volume and infinite density is reached. Now, it is much easier to understand how black holes get their name. As gravitational forces are extremely high, it is impossible for photons generated by their respective black holes to escape such high gravitational force. Hence, no light rays come out of it and thus the name black hole.
Black holes don’t suck anything that comes along their path. Instead only when anything crosses inside the Schwarzschild radius they are sucked by the black hole. Once inside this radius the escape velocity is almost equal to velocity of light.
The Schwarzschild radius can be calculated using the equation for escape speed:
V (escape velocity) = (2GM/R) 1/2
For photons, or objects with no mass, we can substitute c (the speed of light) for V (escape velocity) and find the Schwarzschild radius, R, to be:
R = 2GM/c2
Now an important question arises: If black holes cannot be seen then how are they detected? Whenever black hole passes close to another normal star or interstellar matter, some of the matter is accelerated towards the black hole. This matter in turn gains kinetic energy and heat is generated. The heating ionizes the atoms and at temperature of about few Kelvin X- rays are emitted. This X-ray emitted falls sporadically causing random variation in their intensity. This variation is easily detected and observed.
Among many other candidates, Cygnus X-1 (Cyg X-1) is the longest known of the black hole candidates. It has a highly variable and irregular source, with X-ray emission that flickers in hundredths of a second.