There's new evidence the universe is home to a type of black hole that's not too large and not too small. As black holes go, it's a middleweight that may represent the missing link between its flyweight relatives and the super-heavyweight variety found at the center of most galaxies.

Using NASA's Chandra X-ray Observatory, several groups of scientists have zeroed in on a mid-mass black hole located about 600 light years from the center of galaxy M82. "This opens a whole new field of research," said Martin Ward of the University of Leicester, England, a lead author involved with the observations. "No one was sure that such black holes existed, especially outside the centers of galaxies."

The M82 galaxy got its name nearly 220 years ago when it became the 82nd entry in a systematic catalog of nebulae and star clusters complied by French astronomer Charles Messier.

The black hole found in M82 packs the mass of at least 500 suns into a region about the size of the Moon. Such a black hole would require extreme conditions for its creation, such as the collapse of a "hyperstar" or the merger of scores of black holes.

"This black hole might eventually sink to the center of the galaxy where it could grow to become a supermassive black hole," said Dr. Hironori Matsumoto of the Massachusetts Institute of Technology (MIT), Cambridge, the lead author on one of three Chandra papers scheduled to be published on the mid-mass black hole.

Although previous X-ray data from the German-U.S. Roentgen Satellite, and the Japan-U.S. Advanced Satellite for Cosmology and Astrophysics (ASCA) satellite, suggested that a mid-mass black hole might exist in M82, the crucial breakthrough came when astronomers compared the new high resolution Chandra images with optical radio and infrared maps of the region. They determined that most of the X-rays were coming from a single, bright source.

Repeated observations of M82 over a period of eight months showed the bright X-ray source gradually peaking before dimming. Another critical discovery was that the intensity of the X-rays was rising and falling every 600 seconds.

This flickering of the X-ray intensity is similar to the well-studied characteristics of black holes swallowing gas from a nearby star or cloud," said Dr. Philip Kaaret of the Harvard-Smithsonian Center for Astrophysics, lead author on the paper reporting the 10 minute variations. "Explanations other than a massive black hole for this object are implausible."

Observations with Japan's Nobeyama Millimeter Observatory by Dr. Satoki Matsushita of Harvard-Smithsonian and colleagues have revealed a large expanding superbubble of gas centered on the mid-mass black hole in M82. The energy of several thousand supernovae would be required to produce such phenomena.

In the past, our Milky Way galaxy could have produced mid-mass black holes during periods of vigorous star formation. Hundreds of these massive black holes may exist unseen in our galaxy, in addition to the dozen or so known stellar black holes and the supermassive black hole that is safely confined to the galaxy's nucleus.

Scientists from Kyoto University, Ehime University, RIKEN (The Institute of Chemical & Physical Research), and Nobeyama Radio Observatory, all in Japan, were also involved with the Chandra observations.

The observations were made with the High Resolution Camera (HRC) and the Advanced CCD Imaging Spectrometer (ACIS). The HRC was built for NASA by the Smithsonian Astrophysical Observatory, Cambridge, MA. The ACIS instrument was built for NASA by MIT, and Pennsylvania State University, University Park.