Galactic clash creates its own art form

A work of modern art? An oddly lit spider's web? No: this is one of the latest pictures from the Hubble space telescope, and shows two galaxies colliding - an event which astronomers now think provides the fuel for quasars, the most energetic objects known in the universe.

Discovered 33 years ago, quasars are an enigma because they are so small and yet pour out fantastic quantities of energy: The size of our solar system, they can beam out 1,000 times as much light as entire galaxies containing tens of billions of stars.

Before the latest pictures, astronomers had suspected that the heart of a quasar was a huge black hole - a star which has collapsed under its own gravity, and which sucks in everything around it, including light.

Matter being drawn into the black hole would accelerate, causing it to emit bright light.

Usually a black hole might only feed on interstellar gas. But when galaxies drift into each other - as in this picture, where the differently coloured wraiths each consist of millions of stars - a large black hole will be close enough to drag in entire suns, providing it with the raw material for a super-bright energy source.

"In nearly every quasar we look at we clearly see one galaxy apparently swallowing another," said Mike Disney, of the University of Wales, who is the leader of the team of European scientists analysing the data.

The Hubble telescope has been vital in solving the quasar puzzle, he said. "I gave up on studying [quasars] 20 years ago, when I realised we had to wait for a space telescope to provide a clear enough view for solving mysteries."

However, answering some questions has thrown up new ones. Professor Disney says that the uncertainty has now shifted towards how long a quasar remains bright. If this length of time is short compared with a galaxy's life - 100 million years, or less - then most galaxies, including the Milky Way, could be the leftovers of "burnt out" quasars.