A black hole is a region of space with such a strong gravitational field that not even light can escape.

For most of a star's life, outward pressure from nuclear activity balances the inward force of gravity. When a sun runs out of fuel, gravity compresses the material in the core and it collapses under its own weight.

If a star’s core is massive enough, nothing can stop this collapse and the star is compacted into a single point with zero size and infinite density referred to as the “singularity”.

The gravitational field of a typical black hole is so strong that a person who weighs 100 pounds on Earth would weigh 30 billion tons at the point just before he or she was dragged into the black hole’s inescapable gravity well.

For an object to escape a gravitational field, it must reach escape velocity. On Earth, this equals 7 miles/second (11 km/sec). For a black hole, this velocity reaches the speed of light (186,000 miles/second) at the point of no return.

But why is light, which has no mass, captured into a black hole? Albert Einstein theorized that gravity warps spacetime. Light, which ordinarily travels a straight path, follows a curved path around a gravitational field.

Physicists predict that when gravity distorts spacetime to an extreme, the bottom falls out of the curve, creating the bottomless well we call a black hole. Normal effect of gravity on spacetime. Extreme curvature creating a black hole.

Black holes have their own distinctive anatomy, consisting of: Singularity Event horizon Accretion disk Gas jets

The single point at the center of a black hole where its entire mass is contained is called the singularity.

The event horizon is the rim or boundary of a black hole where escape velocity equals the speed of light. Once inside the event horizon, particles and light cannot escape.

At the event horizon, dimensions as we know them become distorted. To an outside observer, light and time would seem to stand still. An object falling into a black hole would appear to slow and stop at the event horizon.

An accretion disk is a flat sheet of gas and dust surrounding a black hole. This artist’s conception suggests how an accretion disk might appear.

Black holes may have gas jets caused by the interaction of gas particles with strong, rotating magnetic fields. These jets can extend millions of light years through space. Artist’s conception of a black hole with gas jets and an accretion disk.

Astronomers describe three types of black holes:   Supermassive black holes   Stellar black holes   Miniature black holes Artist’s conception

Supermassive black holes can have masses equivalent to billions of suns. They are believed to exist in the centers of most galaxies. Orbiting stars may be captured and their mass added to the black hole. Artist’s conception of a star being drawn into a black hole.

The Hubble Space Telescope has imaged a number of galaxies believed to have black holes at their centers.

Stellar black holes are produced by the collapse of dying stars, and have masses 3 to 10 times that of our Sun.

Astrophysicists believe that miniature black holes might have formed at moment the universe was created, but have no proof of their existence. Miniature black holes have event horizons as small as the width of an atomic particle and contain as much matter as Mt. Everest. Quantum theory suggests that these black holes -- if they exist -- may evaporate over time.

A black hole itself is invisible because no light can escape from it. It can be found indirectly by observing its effect on nearby stars and interstellar gases. How do we know black holes exist?

The image on the right is the signature of a supermassive black hole in Galaxy M84, as seen by Hubble’s Space Telescope Imaging Spectrograph (STIS). The photo on the left shows the slice of space that STIS was analyzing.

Astronomers also measure x-rays from the gas and dust of surrounding stars that are drawn into orbit around the black hole. The gas becomes heavily compressed and friction among the atoms causes x-rays to be emitted. Illustration comparing a black hole and neutron star in x-rays. The black hole does not reflect light shown by its dark center.

Using x-rays, scientists can measure the heat and speed of orbiting material, and from this can detect the presence of a black hole. The mass of the black hole can be determined by the speed of the gas. Chandra x-ray image of galaxy cluster A2104.