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Wednesday, 16 October, 2002, 21:46 GMT 22:46 UK
Galaxy's dark centre exposed
Galactic centre, Yepun/Eso
Yepun view: Arrows point to the position of the black hole
There now seems little doubt that a supermassive black hole resides at the centre of our galaxy, the Milky Way.


These results are the best evidence yet that supermassive black holes are not just theory, but fact

Dr Karl Gebhardt
Scientists say they have produced the best evidence yet to confirm the object's existence.

It comes from observations of a fast-moving star which orbits close to the hole, referred to by astronomers as Sagittarius A* (its location in the sky is in the southern constellation Sagittarius).

Calculations based on the behaviour of the star suggest the black hole has a mass roughly 2.6 million times that of the Sun.

Galaxy evolution

Black holes are among the most exotic phenomena in the Universe.

Yepun, Eso
The advanced optics of the VLT's Yepun unit provided vital data
Theory suggests they are point like objects that have such strong gravitational attraction that all matter that comes too close is sucked in - not even light can escape their influence.

Some are thought to form when dying stars collapse in on themselves. But there is also growing evidence that most if not all galaxies contain other, much bigger black holes at their cores.

How these supermassive objects are created and how they relate to the creation and evolution of galaxies is a mystery.

Realistic explanation

Although the presence of the Milky Way's huge black hole, sited about 26,000 light-years from Earth, has long been suspected, the new data are said to confirm its presence.

"It is a great step forward," said Dr Reinhard Genzel, from the Max Planck Institute for Extraterrestrial Physics near Munich, Germany, a co-researcher on the project team.

"We have been able to exclude some still possible alternative configurations... there is nothing left that one would consider realistic and possible, other than a black hole."

The new evidence is reported in the journal Nature.

Very fast

It centres on 10 years' worth of observations of a star called S2, including studies conducted with the world's largest optical telescope - the VLT - at the Paranal Observatory in Chile.

The data show S2 circles the galactic centre every 15.2 years; its closest approach is a mere 17 light-hours.

"This is the only case we know of in all astronomy where such a star is so close and we can observe it. Most of the other stars have orbital periods between hundreds and millions of years," Dr Genzel said.

S2, which is 15 times more massive than the Sun, survives because it skirts the hole's event horizon - the line inside which matter cannot escape - at an astonishing 5,000 kilometres (3,000 miles) per second.

'Compelling evidence'

"On this trajectory, it is still safe, but if it collides with another star and the orbit changes, it could get sucked in," says Dr Rainer Schoedel, also from the Max Planck Institute and lead author on the Nature paper.

Other scientists are impressed with the latest research.

"These new data probe the galactic centre more closely than ever before," says Dr Karl Gebhardt, a University of Texas astronomer, in a commentary.

"The only compelling explanation is that there is a supermassive black hole lurking there. These results are the best evidence yet that supermassive black holes are not just theory, but fact."

The UK joins the European Southern Observatory.


VLT FORUM

FACT FILE
See also:

19 Dec 00 | Science/Nature
01 Dec 00 | Science/Nature
20 Sep 00 | Science/Nature
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