A mammoth guide to the galaxy

A star cluster is born: New astral bodies may yield clues to the solar system's evolution

It may look like an elephant, but to astronomers this picture is actually the clearest yet taken of the heart of the Milky Way - and they say that it shows our galaxy has a bright future - for a few billion years, at least.

They also hope it will provide definitive data on just how many stars there really are in our galaxy - a number now estimated at 50 billion - and whether the mystery of "dark matter", thought to make up much of the content of the universe but never seen, has been solved.

The shape in the first picture - dubbed "the Mammoth" - shows an area near the core of our galaxy, 20,000 light years away, where stars are just beginning to form. Because the pictures were taken with an orbiting telescope which is sensitive to heat, the brighter areas are hotter - where gigantic dust clouds are collapsing in on themselves, the first step towards becoming a star.

The second picture, of a region named "the Spider", shows an area only 350 light years from the centre of the galaxy, where stars are also being formed.

Because the Earth and Sun lie near the perimeter of the Milky Way, astronomers have always been intrigued by exactly what lies nearer the core, where the stars are much closer together. Some theories have suggested there may be a black hole at the centre of the galaxy, or that the stars are so close together they could spark each other into becoming supernovae.

But they could not previously see what lay there because of the thick dust clouds lying between us and the galactic core. These dust clouds show up as dark patches in the night sky.

These first pictures from the $1bn (pounds 645m) Infrared Space Observatory (ISO) telescope, released yesterday, can penetrate the dust - which is transparent to heat, but not light - to "see" areas of the galaxy that are heating up.

"This shows that the centre of the galaxy is predominantly full of hot, young stars," said Gerry Gilmore, in charge of the ISO project. "In a few billion years it'll become a boring, middle-aged, suburban graveyard of a galaxy, because the stars will run out of fuel. But in the meantime it's going fine."

The real usefulness of the pictures lies in the data they offer astronomers trying to take a census of the Milky Way. "This helps define how many stars are forming, and the structure of the galaxy," said Dr Gilmore. "And that then means we can work out how long the galaxy will last."

The pictures may also yield important information about dark matter - the invisible mass that theory predicts must make up most of the universe. "One of the big questions is whether there are clouds of dark matter at the centre of galaxies," said Dr Gilmore. "This is going to revolutionise our understanding."

Astronomers hope ISO will help to answer another pressing, but difficult question: What shape is the Milky Way? Theory has long held that it is a spiral galaxy, with the solar system located near the end of one of its two arms. But some have argued that it may be cigar-shaped, or just a jumble of stars with no special shape. "We're really looking forward to seeing the answers there," said Dr Gilmore. "Is there a really dense cluster of stars in the centre? How many stars? We should know the answer in a couple of weeks."

The pictures were presented yesterday to an international meeting of astronomers in Birmingham. Dr Gilmore, who had seen the pictures for the first time early on Monday, said they provided " a whole new window on our understanding of the galaxy we live in".