The first VLBA image of the expanding shock wave in RS Ophiuchi. Image: NRAO / AUI / NSF UK and German astronomers have watched the progress of a nuclear explosion on a star 5000 light years from the Earth. But this is just a foretaste of things to come – the same star will eventually be torn apart as a supernova.

In February Japanese astronomers reported an eruption on the star RS Ophiuchi. This is a system where the gravitational pull of a white dwarf (the superdense remnant of a star similar to our Sun) is sucking material from the atmosphere of a nearby red giant. A thick layer of gas builds up on the surface of the white dwarf and every so often this detonates in a violent thermonuclear explosion like that in a hydrogen bomb.

How the RS Ophiuchi system might look at the moment of the February 2006 outburst. Image: David A. Hardy This year’s event was unusually dramatic and RS Ophiuchi briefly became visible to the unaided eye. Scientists then used arrays of radio telescopes to study the aftermath over the next few months – including the UK’s Merlin and the Very Long Baseline Array (VLBA) that stretches from Hawaii to the Caribbean. Images from these systems show an expanding blast wave produced as the explosion hits the red giant companion.

Telescopes at the Jodrell Bank radio observatory in Cheshire. Image: Tim O'Brien, Jodrell Bank Observatory After this outburst is over, gas will once again start to collect on the white dwarf star until another explosion follows, perhaps in 20 years or so. It is possible that not all of the material accumulated is thrown off in the outburst, in which case the white dwarf is slowly increasing its mass. If this happens then the star will eventually be torn apart in a supernova explosion. This would be bright enough to see in the daytime sky and brighter than every celestial object bar the Sun and Moon. The bad news is we have no idea when this might happen – it could be in the next 1000 years or not until 100000 years in the future. But if we were lucky enough to witness this, astronomers would take a keen interest. Most supernovae of this type happen in distant galaxies and scientists would love to see one close by.