dipper@utastro.UUCP (Debbie Byrd) (11/14/86)
If black holes are truly black, how can we detect them? More -- after this. November 14 How to See a Black Hole Take one star that's very massive -- and as it dies let it collapse to an infinitesimal size. The star in effect disappears -- but its gravity remains. Extreme gravity creates a black hole in space -- a place whose inward pull is so strong that nothing, not even light, can escape it. Black holes were first discovered in theory. And yet astronomers believe they can find black holes in space. But how can they see something that doesn't give off light? The answer is that they don't see the black holes themselves. Instead, they see the effect black holes have on the region of space around them. A common model for a black hole depicts two stars in orbit around one another. If one is a black hole, its gravity could be pulling material off of the ordinary companion star. But -- and here's the key -- material from the orbiting star doesn't fall directly into the hole. According to astronomers' calculations, matter spirals down into the hole -- forming a flat disk of gases around an actual hole in the fabric of the universe. The matter spirals toward the hole faster and faster, nearly as fast as the speed of light. Internal friction heats this material to immensely high temperatures -- perhaps billions of degrees! The high temperatures cause the disk to radiate its energy primarily in the form of x-rays. And the x-rays are the giveaway. A powerful source of x-rays coming from a region of space that looks empty points to the likely existence of a black hole. Script by Deborah Byrd. (c) Copyright 1985, 1986 McDonald Observatory, University of Texas at Austin