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Wednesday, 13 September, 2000, 08:55 GMT 09:55 UK
Middleweight black hole discovered
M82 Chandra/Nasa
The X-ray source brightened over time
Astronomers say they have discovered a new type of black hole - one that is of intermediate size.

Until now, scientists have observed only two types of black holes - ones that are about the same mass as stars, and ones that are a million or a billion times bigger than the mass of a star.

"No-one was sure that such black holes existed, especially outside the centres of galaxies," said Professor Martin Ward, of Leicester University, UK, one of the scientists to report the discovery.

"Finding it is like finding the missing bit of a puzzle."

The new type of black hole was located about 600 light years from the centre of the M82 galaxy using Nasa's Chandra X-ray observatory. It is part of a binary system in which the hole and a very much smaller object move around each other.

Single, bright source

The black hole is at least 500 times the mass of our Sun but no bigger than 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 centre of the galaxy where it could grow to become a supermassive black hole," said Dr Hironori Matsumoto of the Massachusetts Institute of Technology (MIT), US, another of the lead scientists who will soon publish details of the discovery.

Previous data from other orbiting observatories had suggested the existence of a mid-mass black hole in M82, but it took new, high-resolution images from Chandra to confirm the identity of a single, bright source of X-rays.

This type of radiation is often the telltale sign for the presence of black holes. The objects cannot be seen directly - gravity is so strong not even light can escape from a black hole - but X-rays are given off by matter torn apart as it falls into the hole.

The Chandra observations showed the X-ray source in M82 to gradually increase its brightness seven fold before dimming. At the same time, the intensity of the X-rays rose and fell every 10 minutes.

X-ray flickering

"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.

"Explanations other than a massive black hole for this object are implausible."

Astronomers think the discovery may have implications for their theories on stellar evolution.

Professor Ward said: "We have some idea of how to form the huge black holes and the small ones, but the process is really rather different, so the question is how do we form these middleweight black holes?

"And actually the answer may well pose challenges for our pre-existing notions of star birth and star death."

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See also:

26 May 00 | Sci/Tech
Chandra homes in on a black hole
06 Jun 00 | Sci/Tech
Black holes blow as well as suck
08 May 00 | Sci/Tech
Black hole search begins
20 Mar 00 | Sci/Tech
Black Holes put on weight
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