What is a Black Hole?

A Black Hole Overflows From Galaxy Centaurus A Image Credit: X-ray: NASA/CXC/CfA/R.Kraft et al.; Submillimeter: MPIfR/ESO/APEX/A.Weiss et al.; Optical: ESO/WFI

We hear about black holes both in science fiction and in popular science literature, but although this astronomical phenomenon is often mentioned, it is seldom explained.

A black hole isn’t really a hole – it is an object that is so highly compressed that its escape velocity exceeds the speed of light.

The result of this ultra-high compression? Nothing can escape.

What does all this mean?

Escape Velocity

Black holes are best understood in terms of their escape velocity.

The escape velocity of a star, planet, moon, or other astronomical object is the minimum speed needed to escape into space from that object.

For example when NASA launches a rocket, there is a minimum speed the rocket needs to escape Earth’s gravitational field, which is Earth’s escape velocity. If a rocket does not reach escape velocity it falls back to Earth.

The escape velocity from the surface of an astronomical object depends on the strength of the gravitational field at the object’s surface. The mathematical formula for escape velocity includes the object’s total mass and radius. If a star collapses to a smaller size while keeping the same mass, it is compressed to a higher density and the escape velocity from its surface increases.

Black Holes: How They Work

How does a black hole form? If a star collapses to an extreme degree, the escape velocity from the surface eventually reaches the speed of light. At this point, the star becomes a black hole. According to Einstein’s special relativity theory, the speed of light is the ultimate speed limit in the universe. Nothing can travel faster than the speed of light. Light, which is massless, travels at the speed of light. Any particle or object having mass can travel close to but never reach or exceed the speed of light. Hence when a star collapses to the point that its escape velocity exceeds the speed of light, nothing can escape.

A black hole is therefore simply a star, or other object, that has collapsed to the point where its escape velocity exceeds the speed of light. Nothing, not even light, can escape from a black hole. Anything that falls into a black hole is trapped forever.

A jet of material originating from the supermassive black hole in the nucleus of the giant elliptical galaxy, M87. Image Credit: NASA/Hubble Heritage Team

Black Holes Are Not Cosmic Vacuum Cleaners

The fact that nothing can escape from a black hole leads to a very common misunderstanding about black holes. Perhaps spurred by popular media, many people think that black holes act as cosmic vacuum cleaners sucking up anything that comes near them.

While it is true that nothing can escape from a black hole, a star, planet, or anything else near a black hole is no more likely to fall into a black hole than it would be to fall into a star of the same mass the same distance away. A black hole has a strong gravitational field at its surface, but at a distance it has the same gravitational field strength as an ordinary star with the same mass. Contrary to popular belief, black holes are not the vacuum cleaners of the heavens.

Black holes have collapsed so much that their escape velocity exceeds the speed of light, so nothing can escape from a black hole. However objects are no more likely to fall into a black hole than into an ordinary star with the same mass.