Black Holes

Taking gravity to the extreme!

We can't see them, but we know that black holes can exist thanks to the groundwork laid by Einstein's General Theory of Relativity. A black hole forms when the mass of an object, like a star, suddenly collapses down to a tiny volume. A small object with a large mass causes a gaping dent in space-time. This enormous warp creates a gravitational field so strong that nothing—not even light—can escape from it.

How do We Know Black Holes Exist?

Astronomers have more than three decades' worth of clues supporting the existence of black holes. Future technology may soon provide even stronger evidence that black holes really are out there.


Einstein's cross: A galaxy's gravitational field redirects light from a quasar to produce four "copies" that form a cross.

Credit: J. Rhoads (STScI) et al., WIYN, AURA, NOAO, NSF

Cosmic Core

Astronomers have evidence that some galaxies orbit supermassive black holes at their cores. By tracking the movements of stars near the center of the Milky Way galaxy, scientists found that the stars orbit a massive invisible object—most likely a black hole about two million times the mass of the Sun.

Ripples in Space-time

When a star collapses to form a black hole, the event sends gravitational waves rippling out through space-time. Detecting certain characteristic waves—with the Laser Interferometer Gravitational Wave Observatory (LIGO)—could provide the first direct evidence that black holes warp space-time.

Finding Black Holes

Astronomers look for clues to locate black holes, but a single clue doesn't confirm that a black hole is really there.

A black hole's gravitational field pulls gas from a companion star and compresses it. This compression raises the gas temperature to such a degree that it emits x-rays.

Astronomers know that if they observe a distant star wobbling, it is orbiting a companion object. If that object is invisible and emitting x-rays, it could be a black hole.

The star's distance from the x-ray source and the speed and magnitude of its wobble indicate the mass of the invisible object.

If the mass of the invisible object is about three times the mass of the Sun or larger—and all the other clues are in place—you've found a black hole!


The Einstein ring: An object that creates a strong gravitational field can block the direct view of a galaxy while deflecting the light the galaxy emits. This gravitational lens can create duplicate images of the galaxy that appear anywhere along an imaginary ring surrounding the lens.

Credit: L. J. King (U. Manchester), NICMOS, HST, NASA

Anatomy of a Black Hole

A black hole forms when a mass collapses down to a tiny volume. When compressed to this extreme, a large mass forms a severe dent in space-time.

event horizon—the point of no return. Anything that crosses this threshold can never escape the black hole's gravity.

singularity—the point in space-time into which a mass collapses. Gravity becomes so strong that our current theories, including General Relativity, break down here.

Schwarzschild radius—named for Karl Schwarzschild, who discovered the relation between the size of the event horizon and a black hole's mass.

The Final Stretch

If you jumped into a black hole, gravity would grow stronger as you fell toward the black hole's center. With your feet deeper inside than your head, gravity's grip would stretch your legs like strands of spaghetti.

The Big Squeeze

Imagine planet Earth compressed to the size of a marble. Such a small object with an enormous mass would create an extreme warp in space-time—a black hole.

The Black Hole Effect

Is something blocking your view? Maybe it's a black hole!

Wavy antique windows and thick eyeglass lenses alter our views by redirecting the path of light from objects to our eyes. As Einstein predicted, gravity creates a similar "lensing" effect. Distant stars and galaxies may appear misplaced, duplicated, or warped because the light they emit changes course as it follows a bend in space-time made by a strong gravitational field. The superstrong gravity of a black hole creates a unique distortion by trapping light in the center of the image while warping the rest of the image around the outside.