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Tuesday, December 8, 1998 Published at 19:20 GMT


The dark heart of our galaxy is revealed

The centre of the Milky Way seen in radio waves (Dot is exact centre)

By BBC News Online Science Editor Dr David Whitehouse

You cannot see the heart of our star system. It is obscured by vast clouds of gas and dust in the constellation of Sagittarius. Try as they might astronomers simply cannot see through these clouds to the centre of our Milky Way galaxy.

Except if they use radio telescopes. Unlike light waves, radio waves are not blocked by the dust clouds. Radio astronomers have now made a new map of the radio sources near our galaxy's core. One of them is the very centre of our galaxy, they believe.

And it is a strange place. Observing the motions of the stars orbiting the central object allows scientists to estimate its mass. It is colossal, 2.5 million times heavier than our Sun but crammed into a small region of space. To astronomers this means one thing - a supermassive black hole.

For the first time, astronomers have determined the size and shape of the highly-charged region of radio emission surrounding the black hole. Indeed the new evidence may force theorists to revise their ideas about how material behaves in the vicinity of black holes.

Using a network of radio telescopes scattered across the USA, an international team of astronomers from the United States and Taiwan studied the area generally thought to mark the galactic centre. This object is called "Sagittarius A-star" (Sgr A*) and is some 26,000 light years from Earth. The researchers think Sgr A* may be an extremely energetic inner region of ionised gas falling into a supermassive black hole.

"However, none of the competing models for a black hole can completely explain both the small size and asymmetrical shape of Sgr A* we have observed," says Jun-Hui Zhao, a member of the team from the Harvard-Smithsonian Centre for Astrophysics.

The nature of Sgr A* has been a long-standing puzzle in astronomy since its discovery in 1974. Since then, there have been many theories about the structure and emission mechanism of Sgr A*. In the past few years, astronomers have found increasing evidence that it is a supermassive black hole.

But despite being millions of times more massive than our own Sun, the black hole at the centre of our galaxy would be relatively small in cosmic terms. Its event horizon, the outer edge inside which not even light can escape from the black hole's gravitational pull, would be only about 7.5 million kilometres. That is only a twentieth of the mean distance between the Earth and the Sun.

The astronomers are particularly intrigued that the radio-emitting area immediately surrounding the event horizon has an elongated shape. At the moment the shape of the radio-emitting, cigar-shaped gas cloud at the core of the galaxy is a mystery. Further computer simulations of our Milky Way's central regions are planned to see if it can be explained.

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