Detection Of Black Holes
In their near vicinity, black holes produce bizarre effects; from a distance, however, they are well-behaved. If one were to replace the Sun with a black hole of the same mass as the Sun, there would be a region of space a few kilometers in size, located where the center of the Sun currently resides, in which space would be extremely warped. The gravitational field of this object, measured at the distance of the earth, would be exactly that of the present-day Sun. The earth and planets would continue in their orbits and the solar system would continue much as it does today—only in the dark.
Normally, an observer must get within a few Schwarzschild radii in order to feel the distinctive effects of the black hole. Indeed, one of the observational tests for the presence of a black hole in binary systems is to look for the characteristic radiations of matter being heated as it is squeezed during its final plunge toward the black hole's event horizon. Such matter will emit fluctuating x rays as a result of being squeezed. The rate of fluctuation is tied to the size of the emitting region. Astronomers find that in such systems the x rays come from a volume of space only a few kilometers in diameter. In several instances, analysis of the orbital motions in a binary star system with only one visible member (a conventionally shining star) indicates that the dark, unseen member of the binary system is much more massive than the Sun. A dark stellar component more massive than the Sun confined to a volume smaller than a few kilometers is a prime candidate for a black hole.
There is at least one other situation in which astronomers suspect the existence of a black hole. Because a black hole that is not actively swallowing large amounts of matter does not radiate significantly, we must detect it indirectly, through the effect of its gravitational field on neighboring objects. In the centers of many galaxies, the stars, gas, and dust of the galaxy are moving at very high speeds, suggesting they are orbiting some very massive, comparatively small object. If the object was a tightly packed collection of massive stars, it would shine so brightly as to dominate the light from the galactic center. The absence of light from the massive, central object suggests it is a black hole. Recent astronomical observations have confirmed many galaxies, including our own, have supermassive black holes at their centers. Scientists believe that all galaxies may be organized around such black holes; after the Big Bang, supermassive black holes may have formed first, then gathered the galaxies around them.
In one galaxy, the Hubble Space Telescope (HST) has photographed a spiraling disk of matter that appear be accreting onto a central massive dark object that is likely to be a black hole. Recently a large team of astronomers reported the results of a worldwide study involving the HST, the International Ultraviolet Explorer satellite, and many ground based telescopes. Instruments were able to detect light that was emitted by the accreting matter as it spiraled into the black hole that was subsequently absorbed and re-emitted by the orbiting clouds just a few light-days away from the central source. Mass estimates of the central source determined from the motion of these clouds suggests that the object has a mass of at least several million times the mass of the Sun. So much material contained in a volume of space no larger than a few light-days in diameter provides some of the clearest evidence yet for the existence of a black hole at the center of any galaxy.
In another study with the HST and a ground-based telescope in Hawaii, scientists were able to observe a black hole in a two-star system in the constellation Cygnus. This black hole is sucking material from its companion star in a swirling disk of material and hot gases, swallowing nearly 100 times as much energy as it radiates. The material being pulled in toward the black hole stores its energy as heat until the critical moment. The observations show gas at temperatures of over a million degrees falling toward the event horizon of the black hole.