London, Oct 3 (PTI): Astronomers have discovered seven supernovae in a galaxy nearly 250 million light years away from Earth, confirming the belief that the universe's most efficient star factories are also supernova factories.

A team at Chalmers and Onsala Space Observatory in Sweden says that never before have so many supernovae been spotted at the same time in the same galaxy.

The astronomers used a worldwide network of radio telescopes in five countries, including Sweden, to be able to create extremely sharp images of the galaxy Arp 220. They observed around 40 radio sources in the centre of the galaxy.

These radio sources are hidden behind thick layers of dust and gas and invisible in ordinary telescopes. To discover the nature of these radio sources, they made measurements at different radio wavelengths and watched how they changed over several years.

"With all the data in place, we can now be certain that all seven of these sources are supernovae: stars that exploded in the last 60 years," said Fabien Batejat, main author of the article about the discovery.

The number is nevertheless consistent with how fast stars are forming in Arp 220.

"In Arp 220, we see far more supernovae than in our galaxy. We estimate that a star explodes in Arp 220 once every quarter. In the Milky Way, there is only one supernova per century," said Rodrigo Parra, astronomer at the European Southern Observatory in Chile and member of the team.

Added John Conway, another team member: "Arp 220 is well-known as a place where star formation is very efficient. Now we have been able to show that star factories like this are also supernova factories."

The findings are to be published in an upcoming edition of the 'Astrophysical Journal'.