Event horizon defines a boundary surrounding a black hole, within which the escape velocity exceeds the speed of light. Since light photons cannot escape, events occurring inside the event horizon cannot be seen. So, how do astronomers detect a black hole then? The enormous gravity of the black hole attracts any nearby matter close to it. As this matter falls over the event horizon, it forms a whirlpool-like disk around the event horizon, known as an accretion disk. Just outside the event horizon, the speed of matter is just slightly less than the speed of light. Particles moving with relativistic speeds interact with each other, thereby increasing the temperature of the accretion disk. At very high temperatures, the matter emits X-rays, which the astronomers can detect using the space-based telescopes. A supermassive black hole at the center of a galaxy can be detected by observing the orbital period of the stars close to it. Using Kepler's third law of planetary motion, the mass of such a black hole can be estimated.