X-Ray "Vision" Unlocking Black Hole Mysteries

John Roach
for National Geographic News
May 24, 2005
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 galaxies—phenomena 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 systems—systems 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.

A second type of black hole is the supermassive variety, which is thought to exist at the center of each galaxy, including the Milky Way. These black holes can be detected by the way their gravity distorts nearby stars, said Morgan, the MIT astrophysicist.

Though scientists are uncertain how supermassive black holes form, they appear to be closely related to the growth of galaxies.

"For example, many astronomers have recently found that the huge black holes at the centers of galaxies undergo growth spurts at the same time as the galaxies themselves are having growth spurts," Edmonds said.

Better Resolution

The future of black hole detection may be even brighter if NASA moves forward with its proposed Nuclear Spectroscopic Telescopic Array (NuSTAR).

Current x-ray telescopes are limited to detecting relatively low-energy x-rays, which are more easily blocked by gas and dust than are high-energy x-rays. NuSTAR will be able to detect significantly higher-energy x-rays, said Fiona Harrison, an astrophysicist at the California Institute of Technology in Pasadena.

"As you go to higher and higher energy, you are capable of uncovering those black holes that are surrounded by dust and gas. We know there are a lot of those out there," said Harrison, who also serves as NuStar's principle investigator.

The proposed array is in the final stage of a review process. A final funding decision is due in February 2006. If approved by NASA, NuSTAR would launch in February 2009.

The array would survey and reveal black holes in areas of the sky previously examined with the less-powerful Chandra X-Ray Observatory. NuSTAR should help astronomers understand how black holes came to be in the universe and how they have grown over cosmic time.

"Every large galaxy has a black hole in the center and we are trying to understand this process," Harrison said. "They are turning out to be quite important in forming structure in the universe."

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