A black hole is defined as a region of spacetime from which huge gravitational attraction prevents anything, including light, from escaping. If space-time is like a trampoline, a star causes a gravitational dent in the trampoline bed – the heavier it is the more the bed is distorted. A black hole distorts the bed to a massive degree, so much so that the ‘hole’ made by the hugely dense body has no bottom – like a tube with no end – a singularity in other words. General relativity predicts that a sufficiently compact, dense mass will deform spacetime to form a black hole. Around a black hole, there is a mathematically defined surface called an event horizon that marks the point of no return. An event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. As water drains down a plughole, there is a radius less than from which a small floating object on its surface can no longer escape, in the same way that a ball rolled diagonally across the trampoline bed cannot escape from falling down the hole no matter how fast it is going, or light cannot escape from the curved fabric of spacetime. The hole is called “black” because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics. Similarly, any object approaching the event horizon from the observer’s side appears to slow down (time dilation) and never quite pass through the horizon, with its image becoming more and more redshifted as time elapses.
Event horizons emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole.
The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is the radius of a sphere such that, if all the mass of an object is compressed within that sphere, the escape speed from the surface of the sphere would equal the speed of light. An example of an object smaller than its Schwarzschild radius is a black hole.
Black holes are expected to form when very massive stars (> 40 solar masses) collapse at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing or accreting mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. Supermassive black holes are thought to exist in the centres of most galaxies since many invisible but massive objects appear to co-rotate around a visible binary companion.
This black hole is located in the nearby dwarf galaxy IC 10, 1.8 million ly from Earth in the constellation Cassiopeia. We can measure the black hole’s mass because it has an orbiting companion: a hot, highly evolved star. The star is ejecting gas in the form of a wind. Some of this material spirals toward the black hole, heats up, and emits X-rays before crossing the point of no return.