Einstein's 1915 general relativity theory has predicted the existence of black holes and event horizons. But until recently, telescopes were too imprecise to tell the difference between dim neutron stars and even dimmer black holes.

Now, using a telescope with finer resolution, Garcia and his colleagues have found that black holes are 100 times darker than neutron stars.

Even four years ago, Garcia said, the data on black holes and event horizons would have failed the informal "thumb rule" in science: "If you can stick your thumb on it and blot out the important data point, then you're really in trouble."

Up to that time, technical limitations meant scientists had only identified one x-ray nova as definitely containing a black hole. But new technologies have shown Garcia and his fellow researchers three more x-ray novae with black holes.

"The old results were like that. You could have blotted out that one point," he said. "Now you can't. You can't throw out a point and say it's an oddball."

According to relativity theory, very massive stars--about three times heavier than the sun--eventually collapse to become black holes. In the past, astronomers had assumed that the extremely massive objects in x-ray novae were black holes.

By demonstrating the presence of event horizons, the Harvard researchers say that they have supplied more conclusive evidence that black holes resemble the predictions of Einstein's theory.