Bubble reveals secrets of how stars form

British astronomers have looked deep inside a distant galaxy for the first time to unravel the mystery of how stars are made.

Their observations revealed a gigantic magnetic pump funnelling cosmic dust into the centre of the galaxy, where it piles up until it forms a ball dense enough to spark a nuclear chain reaction. Jane Greaves, the leader of the four-member team, said it was like having an ultrasound scan that shows how stars are formed in a galactic womb.

The galaxy Dr Greaves studied, M82, is 11 million light years from Earth in the constellation Ursa Major, the Plough. It can be seen from Britain with powerful binoculars or a good amateur telescope.

M82 is going through a phase known as a "starburst", producing 50 times as many new suns as our own Milky Way does. Dr Karen Wills, a researcher at the University of Sheffield, said: "It seems likely that all galaxies, including our own, have gone through a starburst phase at some point. So learning about M82 allows us to find out about the way in which galaxies have evolved from the early Universe to the present day."

Until now, astronomers have been unable to see what goes on in other galaxies because their view is blocked by cosmic dust. But Dr Greaves and her team used the faint heat radiation from the dust itself to chart M82's inner workings.

Their computer-enhanced photographs show a huge magnetic bubble, 3,000 light years (18,000 billion miles) across in the centre of the galaxy. Its structure is revealed by the glowing dust motes that have been pulled on to its force lines, like iron filings around a bar magnet.

The bubble is strikingly different from those seen in normal galaxies; spirals like the Milky Way have well-ordered, flat magnetic fields, or plumes of magnetic force lines spouting from their poles. "We were really surprised to see the huge bubble," said Dr Greaves. "This is a new feature of galaxies that we didn't know about before and could show how magnetic fields help shape the evolution of starburst regions."

Exactly how the magnetic field was formed is still not known, but a near-collision with another galaxy, M81, is thought to have been responsible. Although galaxies can pass through each other without any individual stars hitting each other, even a close call involves massive tidal forces that can stir up the dust to set off a starburst.

Dr Greaves' team at the James Clerk Maxwell telescope in Hawaii used a sub-millimetre camera called Scuba - which takes pictures in wavelengths between infrared and microwave - and a polarimeter, which works on the same principle as polarised sunglasses.

At present, the camera is not sensitive enough to pick out individual stars being formed, but a new camera being developed by the Royal Observatory Edinburgh will be 10 times as accurate, allowing Dr Greaves to see concentrations of dust just a few light years across.