# Structure of a Black Hole

Although black holes come in a variety of masses and sizes, their structures are all alike. A black hole's entire mass is concentrated in an almost infinitely small and dense point called a singularity. This point is surrounded by the event horizon - the distance from the singularity at which its escape velocity exceeds the speed of light. And a rotating black hole is surrounded by the ergosphere, a region in which the black hole drags space itself.

The singularity forms when matter is compressed so tightly that no other force of nature can balance it. In a "normal" star, like the Sun, the inward pull of gravity is balanced by the outward pressure of the nuclear reactions in its core. In the collapsed stars known as white dwarfs or neutron stars, other forces prevent the ultimate collapse.

If there is too much mass in a given volume, though, the object reaches a critical density where nothing can prevent its ultimate collapse to form a black hole.

Because gravity overcomes the other forces of nature, a singularity follows its own bizarre rules of physics. Time and space as we know them are crushed out of existence, and gravity becomes infinitely strong.

As the distance from the singularity increases, the escape velocity decreases. Escape velocity is the speed at which an object must move to get away. For Earth, the escape velocity is around seven miles (11 km) per second. In other words, a spacecraft must go at least that fast to escape Earth's gravitational pull and travel to another planet.

At a certain distance from the singularity, the escape velocity drops to the speed of light (about 186,000 miles/300,000 km per second). This distance is known as the Schwarzschild radius, in honor of Karl Schwarzschild, who first defined it. This radius depends on the mass of the black hole. For a black hole as massive as the Sun, the radius is about two miles (3 km). For every extra solar mass, the radius increases by two miles.

This radius enfolds the singularity in a zone of blackness - in other words, it makes a black hole black. It gives the black hole a visible surface, which is known as the event horizon. This is not a solid surface, though. It is simply the "point of no return" for anything that approaches the black hole. Once any object - from a starship to a particle of light - crosses inside this horizon, it cannot get back out. It is trapped inside the black hole.

Anything that enters the black hole increases its mass. And as the mass goes up, the size of the event horizon gets bigger, too. So if you feed a black hole, it gets fatter!

If the black hole doesn't rotate, then its gravitational influence on its environment is straightforward. If the black hole is spinning, though, then its gravitational effects are more complicated. It actually pulls the fabric of spacetime along with it - an effect called frame dragging. This area is known as the ergosphere. Seen in cross-section, it is oval-shaped, with the region of influence extending farther into space at the black hole's equator than at its poles.