Black holes are formed when there is a concentration of mass in space that is so dense, the forces it exerts on nearby objects can not be resisted. This occurs when a massive star (at least 20 times the mass of Earth's sun) reaches the end of its life cycle. Stars are essentially engines in which there is a constant tug of war between nuclear reactions pushing energy outward and gravitational forces pulling matter toward its center. When these forces are equal, the star is stable. But once a star depletes its nuclear fuel, gravity takes over and compacts all of its material into a dense center, eventually creating a black hole.
To understand the pull of a black hole on light, consider the concept of escape velocity. No one has the power to throw a baseball into the air with enough force to escape the gravitational pull of the earth. If the earth were smaller in size, the escape velocity needed to overcome gravity would be less because the mass creating the gravitational pull would be less. If the ball were thrown with enough velocity, it would escape gravity and keep rising into space.
Black holes can be thousands of times as big as Earth. Their gravitational pull is immense. Even the speed of light is too slow to achieve escape velocity against a black hole's attraction. If a space ship were able to achieve light speed and arrived just outside the event-horizon, or surface, of a black hole, it could theoretically break free because the escape velocity would be less than light speed. However, if it passed the event horizon and into the black hole, the ship would be helplessly drawn toward its singularity, or center, because the escape velocity needed would be greater than light speed.
Because light can not escape their attraction, black holes are not visible for study without special viewing equipment. With the Hubble telescope, scientists have determined the presence of black holes by their distortion of gas and light around them from other stars. The ability of an object's gravity to bend light is called gravitational lensing, and refers to Einstein's Theory of Relativity. If a person could sit on one side of a black hole, peering over its event horizon at stars on the other side, the hole's gravity would curve the light from those stars; they would appear stretched and bent like images in a fun house mirror.