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If there were monsters in space, they might appear as black holes - bottomless pits of gravity that rip holes in the fabric of time and space, swallowing up entire stars. Nothing - not even light - can escape. 

The study of black holes began in the 1930's, when an Indian astrophysicist named Subramanian Chandrasekhar found that there is a limit to how much gravity can be supported by matter as we know it. Nothing here on Earth comes close to exceeding this matter-gravity limit, but the phenomenon often occurs in space. 

Every single second, in the middle of a star, violent, uncontrolled fusion reactions occur that are millions of times more powerful than our entire nuclear arsenal. The star's massive gravitational field keeps it from exploding under the pressure. But when a star uses up its nuclear fuel, the mass and gravity continue to press down toward its centre. 

The central portion of a relatively small star like our Sun is then crushed into a hot, super-dense cinder as small as Earth, with maybe half the mass of the Sun. The gravity force squeezes this "white dwarf" star until each teaspoon of material weighs about as much as an 18-wheel truck, but at that point, the crush of gravity stops. For stars somewhat larger than the Sun, the remnant core can exceed about one and one-half Suns (the Chandrasekhar limit for white dwarfs). Gravity is so strong in this case that the collapse continues until the cores are the size of a small town and each teaspoon of their matter weighs as much as one million 18-wheelers. These are the neutron stars, which we often see as pulsars.