Using ESA's XMM-Newton space telescope, astronomers have uncovered the first close-up of a white dwarf star circling a companion star that could explode in a dramatic supernova event in a few million years.

The mysterious object was first noticed in 1997 when astronomers discovered X-rays emanating from around the bright star HD 49798. Using XMM-Newton the celestial enigma has been tracked along its orbit, revealing some curious properties. While most white dwarfs pack 0.6 solar masses into an object the size of Earth, this particular example contains at least double that mass within a diameter half that of Earth. It also rotates once every 13 seconds, the fastest of any known white dwarf.

Astronomers Sandro Mereghetti and collaborators suggest that the white dwarf has grown to its huge size by feeding off gas from a companion star. But at 1.3 solar masses, the white dwarf is now close to a dangerous limit; theory states that once this mass exceeds a critical limit of 1.4 solar masses it will either explode or collapse to form an even more compact object called a neutron star.

The explosion of a white dwarf is the leading explanation for type Ia supernovae, bright events that are used as standard candles to measure cosmic distances, and ultimately help measure the expansion of the Universe.

The discovery is the first time the mass of an accreting white dwarf in a binary system has been determined so accurately. "This is the Rosetta stone of white dwarfs in binary systems," says Mereghetti. "Our precise determination of the masses of the two stars is crucial. We can now study it further and try to reconstruct its past, so that we can calculate its future."

The astronomers predict that the star will explode in a few million years, blazing with the intensity of the full Moon – so bright that it will be seen in the daytime sky with the naked eye.

The new results are presented in the 4 September issue of the journal Science.