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Denver 2003 Monday, 17 February, 2003, 20:41 GMT
The big and the bizarre
Sgr A*, Nasa/CXC/MIT/F.K. Baganoff et al
The Chandra telescope took this image of the hole region
The more they look, the more astronomers are amazed at what they are finding out about the heart of our galaxy.

It's hard to understand how such stars can form so close to a black hole

Prof Andrea Ghez
This is the location of a supermassive black hole, referred to by scientists as Sagittarius A* (its location in the sky is in the southern constellation Sagittarius).

For the first time, researchers have been able to study in detail the light coming from a star that skirts close in to this exotic object at jaw-dropping speeds.

From what the team can discern, their spectral data challenge some theories of how stars form.

'Death pull'

The team leader, Professor Andrea Ghez, spoke about the extraordinary environment that exists at the galactic core to the annual meeting of the American Association of the Advancement of Science in Denver.

She and her colleagues have been using the Keck Observatory on Hawaii to obtain unprecedented views of the region of space around Sgr A*.

Their new data provide the best estimate yet of the mass of the black hole and the extent of its event horizon - the line inside which matter (and even light) cannot escape the death-pull of the hole.

"One star in particular has come within 68 AU - AU being the Earth-Sun distance," she told BBC News Online.

"Sixty-eight AU is about the distance between the Sun and Pluto - so we confine the mass of the black hole to this volume."

That mass is now said to be three million times that of own Sun - give or take half a million. This is the most precise calculation produced so far.

Better resolution

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

Theory suggests they are point like objects that have such strong gravitational attraction that all matter that comes too close is sucked in.

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.

The only way to "see" a black hole is to study the objects that move close in around it.

"To be able see these stars that are the keys - you want to see fast-moving stars close to the black hole - you need high-resolution imaging. Our atmosphere limits our ability to get that resolution but now we have adaptive optics."

One star was seen to move at 9,000 kilometres per second - "that's quite a ride," said Professor Gehz.

Turbulent atmosphere

The mirrors on the 10-metre telescopes at the Keck Observatory have flexible mirrors that can compensate for the distortion light experiences as it travels through the Earth's atmosphere.

This adaptive optics system gives pin-sharp images of the close-in stars allowing Gehz to track accurately their obits and, for the first time, reveal useful information about their chemical composition.

"Today, the big breakthrough is that we're no longer having to average stars together - we can see the full orbit of individual stars which actually confines the mass of the black hole to a volume that's 10 thousand times smaller than what we could do before."

One star in particular has now come in for close scrutiny - it is called S0-2. This star is 15 times more massive than our Sun; it is a thousand times brighter; has a surface temperature of about 30,000 Celsius but is probably less than 10 million years old.

Strange place

Gehz, from the University of California-Los Angeles, says it is a puzzle that so big and so young a star can evolve so close to a supermassive black hole.

"The ability now to get spectra, which is only possible with adaptive optics, has revealed what kinds of stars these are, and it turns out these stars are very massive and very young and it's hard to understand how such stars can form so close to a black hole.

"We need to know if the black hole affects the way these stars appear - maybe they are just older stars masquerading as younger ones."

There is a second team studying Sagittarius A* with Europe's Very Large Telescope facility in Chile. Ghez says the competition is proving very useful in confirming data and revealing the secrets of the strangest place in the Milky Way.

Details on S0-2 by Ghez and colleagues have been submitted to a journal for publication.

Denver, BBC

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See also:

16 Oct 02 | Science/Nature
07 Jan 03 | Science/Nature
17 Oct 00 | Science/Nature
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