Harvard researchers have found what they call the most compelling evidence yet for the existence of black holes and the "event horizons" that surround them, confirming with actual observations a result predicted 86 years ago by Einstein's theory of general relativity.

The findings, announced at the annual meeting of the American Astronomical Society last week, confirmed the existence of event horizons, the point of no return beyond which black holes consume matter.

Once matter crosses that point, nothing can escape--not even light. That means that black holes really are black after all, as theory says they should be.

"Showing that they're black hadn't been done before. It's a fundamental property of black holes," said Michael R. Garcia, a researcher at the Harvard-Smithsonian Center for Astrophysics.

Researchers looked at x-ray novae, which are pairs of stars that rotate quickly and very close to each other. One of the stars in an x-ray nova is a normal star, like the sun, and the other one is a dense, compact star.

In some x-ray novae, the dense star is a neutron star. In these situations, gas and other matter flow away from the normal star and eventually collide with the surface of the neutron star--causing an explosion that releases a bright burst of energy.

In other x-ray novae, the dense star is actually a black hole, a place where all of the star's mass has collapsed to a single point with no volume but an infinitely strong gravitational pull.

In this case, matter from the normal star approaches a so-called "event horizon" that surrounds the black hole. Instead of causing a bright explosion, the matter gets devoured by the black hole. Once something crosses the horizon nothing can escape--not even light.