NEUTRON STARS AND PULSARS
HOW THEY FORM
HOW WE DETECT THEM
Q & A PAGES
Once a giant star dies and a black hole has formed, all its mass is squeezed into a single point. At this point, both space and time stop. It's very hard for us to imagine a place where mass has no volume and time does not pass, but that's what it is like at the center of a black hole.
The point at the center of a black hole is called a singularity. Within a certain distance of the singularity, the gravitational pull is so strong that nothing--not even light--can escape. That distance is called the event horizon. The event horizon is not a physical boundary but the point-of-no-return for anything that crosses it. When people talk about the size of a black hole, they are referring to the size of the event horizon. The more mass the singularity has, the larger the event horizon. The structure of a black hole is something like this:
Many people think that nothing can escape the intense gravity of black holes. If that were true, the whole Universe would get sucked up. Only when something (including light) gets within a certain distance from the black hole, will it not be able to escape. But farther away, things do not get sucked in. Stars and planets at a safe distance will circle around the black hole, much like the motion of the planets around the Sun. The gravitational force on stars and planets orbiting a black hole is the same as when the black hole was a star because gravity depends on how much mass there is--the black hole has the same mass as the star, it's just compressed.
Black holes are truly black. Light rays that get too close bend into, and are trapped by the intense gravity of the black hole. Trapped light rays will never escape. Since black holes do not shine, they are difficult to detect.