for National Geographic News
Advances in x-ray astronomy are resolving some enduring mysteries about black holes, scientists say. Black holes are places in space where the force of gravity is so strong that nothing, not even light, can escape.
In recent years scientists have learned to find black holes by sweeping the skies with space-based telescopes equipped with x-ray "vision." X-rays are a high-energy form of light that is invisible to the human eye.
"As [matter] falls down into the black hole, it will heat up, and it gets so hot it emits x-rays," explained Edward Morgan, an astrophysicist at the Massachusetts Institute of Technology (MIT) in Cambridge.
Morgan is an instrument scientist for NASA's Rossi X-Ray Timing Explorer satellite. Launched in 1995, the satellite allows scientists to study black holes and other objects such as neutron stars.
The NASA satellite and other instruments are helping researchers to uncover details about the final stages of massive stars as well as the controlling forces at the center of galaxiesphenomena related to black holes.
Black Hole Types
Scientists believe there are two types of black holes. The most common were remnants of stars that are at least ten times the size of our sun. These defunct stars have collapsed in on themselves, and as they get denser, their gravitation pull increases.
Eventually the gravity from these stellar remnants gets so strong that not even light can escape it. Astronomers believe most galaxies have millions of such black holes.
All of the known stellar-remnant black holes found in multiple star systemssystems in which two or more stars orbit around a common center of gravity. Multiple-star systems are thought to account for about half of all the stars in the Milky Way, the galaxy that contains our own solar system.
In a binary star system where one star is normal and the other is a black hole, "matter will flow off the star onto the black hole," Morgan said. As it flows onto the black hole, matter spirals like water going down a drain, heats due to friction, and emits x-rays.
But as Peter Edmonds, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, explained, "If there is no matter to fall into a black hole, then the black hole would indeed be invisible."
Edmonds is the press scientist for the Chandra X-Ray Observatory. Launched in 1999 by the Space Shuttle Columbia, the space-based observatory has the sharpest vision of any x-ray telescope, making it the state of the art in black hole detection.
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