| MadSci Network: Astronomy |
The luminescent material you have seen in traditional illustrations of black holes, and, more recently, in actual photographs taken with the Hubble Space Telescope, is matter falling into the black hole. Called an accretion disk, it's heated so much that it gives off visible light, ultraviolet rays, and x-rays. It owes its disk-like shape to its origin, which would be, theoretically, matter coming from a companion star, and also to a lesser extent from other debris, like planets. Many stars (more than half) are in binaries, and the massive stars are even more likely to be binary members - massive stars which may eventually become black holes. When close binaries form, the pair of stars is separated by less than the width our own Solar System (defined by Pluto's orbit), say. Tidal forces tend to align the stars' rotation axes and their mutual plane of revolution about their common center of mass. Thus, matter that flows between them is generally confined to a disk.
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