A team led by astronomers at Chalmers and Onsala Space Observatory has detected seven previously unknown supernovae in a galaxy 250 million light years away.Never before have so many supernovae been discovered at the same time in the same galaxy. The discovery proves what astronomers have long believed: that the galaxies which are the universe\'s most efficient star-factories are also supernova factories. 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. The scientists observed around 40 radio sources in the center of the galaxy Arp 220. 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,\" says Fabien Batejat, main author of the article about the discovery. So many supernovae have never before been detected in the same galaxy. 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,\" says Rodrigo Parra, astronomer at the European Southern Observatory in Chile and member of the team. John Conway is professor of observational radio astronomy at Chalmers and deputy director of Onsala Space Observatory. \"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,\" he says. The radio measurements have also given researchers insight into how radio waves are generated in supernovae and their remnants. \"Our measurements show that a supernova\'s own magnetic field is what gives rise to its radio emission, not the magnetic fields in the galaxy around it,\" says Fabien Batejat. The results will be published in the October 20 issue of the journal Astrophysical Journal.