Even though it is based on Einstein's general relativity equations, the concept strains belief. If a star is large enough —at least three times as massive as the sun—it will eventually die in a grand cataclysm. As its nuclear fires begin to burn out, the stellar gases, no longer supported by heat and radiation, begin falling toward the star's core. Moving at tremendous velocities, they crush together, forming a sphere only two or three miles across, so dense that each cubic inch of material weighs trillions of tons. The small sphere has a gravitational field so strong that no radiation —even light—can escape from what has become a totally invisible "black hole."
If the theory is correct, there could be countless black holes among the billions of stars in every galaxy. But if no light or other radiation can escape from the bizarre objects, how can astronomers prove that they really exist? The answer may lie in the constellation Cygnus (The Swan), where scientists are now almost certain that they have located a black hole. Its presence was hinted at in 1971 by the first earth-orbiting X-ray satellite Uhuru, which detected a strong and widely fluctuating flow of X rays from Cygnus. Scientists suspected that the radiation source, which they named Cygnus Xl, was a pulsar, or neutron star, the result of a different form of stellar collapse. But the uneven fluctuations bore no resemblance to the steady bursts of radiation from other pulsars.
Shortly thereafter, radio astronomers, using their more sharply focused antennas, picked up radio signals from the area. That gave a much more precise fix on Cygnus Xl, letting other astronomers train big optical telescopes on the site. There they found a huge star, a so-called class-B supergiant, at least 20 times as massive as the sun. It was traveling erratically through space, as if it were being tugged by a smaller companion star moving around it. From this gravitational pull, astronomers figured that the unseen star had at least three times the mass of the sun.
Was the invisible companion a black hole? In 1967 Soviet theoreticians had suggested that if a black hole were orbiting a larger, visible star, it would draw gases from the star. As those gases spiraled toward the black hole, they would collide, compress and heat up to as high as 100 million degrees—enough to produce an intense flow of X rays. Recent findings by NASA'S new Copernicus earth satellite strongly support this scenario. Cygnus X-l shows a sharp decrease in X-ray emissions every 5.6 days. That, according to optical astronomers, seems to be the time it takes the bright star's unseen companion to make one trip around it. In other words, every 5.6 days the black hole passes behind the visible star. Thus, the supergiant partially blocks the X rays, resulting in the fluctuations observed on earth.
The combined calculations and observations add up to the best proof yet that a black hole has been found.