Stars And Planets: January

BROTHER CADFAEL might be the best person to assist astronomers perplexed by the "mystery of the missing medieval supernova". It seems that an exploding star of the 13th century should have been as brilliant as the full moon - yet no one reported seeing it.

The first clues to this ancient event came late in 1998, when the Rosat orbiting observatory picked up an unexpected source of natural X-rays in the constellation Vela. This hot bubble of gas looked like the remains of a supernova (an exploding star). Astronomers already knew hundreds of these "supernova remnants", so the new discovery at first looked like just one more for the list.

But this supernova remnant, known only by the catalogue number J08520- 462, proved to be rather unusual. First, it was so small and hot that the explosion must have taken place very recently in astronomical terms, where most things take millions of years to happen. Secondly, Bernd Aschenbach, of Germany's Max Planck Institute for Extraterrestrial Physics, has found that this supernova remnant lies comparatively near the Sun, only a few hundred light years away (right on our doorstep, in a galaxy which is 100,000 light years across).

When a supernova explodes, it shines more brilliantly than 500 million suns, and for several weeks. Aschenbach and his colleagues calculate that the supernova that blew the hot bubble J08520-462 should have appeared in the century AD1200-1300, and it would have shone as brilliantly as the full moon: bright enough to read a newspaper by, if newspapers had been invented then.

Did such a star actually appear in our skies? There are no records of such a brilliant object from Western Europe, even though Brother Cadfael's colleagues in a Swiss monastery did record a dimmer supernova in the year AD1006. Even more telling, Chinese astronomers of the time scrutinised the sky every night for astrological portents. They reported half a dozen supernovae, none anywhere as bright as the full moon. As well as the supernova of 1006, the Chinese saw exploding stars in 1054, 1181, 1572 and 1604. The remains of all these supernovae have been identified, and none of them was responsible for the remnant J08520-462.

We can certainly conclude there was no star as bright as the full moon in the 13th century. Paul Murdin, of the British National Space Centre, says these negative sightings are clues as valuable as Sherlock Holmes's "dog that didn't bark in the night". They prove that a star can explode without a brilliant display of light.

Most supernovae are stars at the end of their lifetime. As the core at the very centre collapses, the outer parts of the star detonate and blast out into space. A few supernovae are something different: a small star, called a white dwarf, that blows apart like a massive cosmic nuclear bomb. Both are brilliant sights.

Murdin suggests the explosion that gave rise to J08520-462 must have been different. Perhaps it was the explosion of a star even smaller than a white dwarf, of a type known as a neutron star. A neutron star might put all its energy into flying shrapnel, instead of a brilliant fireball. Or it might have been a different kind of explosion altogether. Perhaps a neutron star was engulfed by a neighbouring black hole: the energy-packed contents of the neutron star erupted into space, but little light escaped before the star's remains were swallowed up by the black hole.

So far, no definitive answer has emerged. But, one way or the other, the silence of medieval records is pointing us towards new discoveries at the cutting edge of astronomy.

WHAT'S UP: There are two full moons this month. Strictly speaking, a month is the period the moon takes to wax and wane - the word itself derives from "moonth". But the interval between full moons is 29.5 days, so a calendar month can occasionally contain two full moons: the last one was July 1996.

In the first week of the New Year, look out for the Quadrantid meteors, which reach a maximum on the night of 3-4 January. These shooting stars spread out from a point near the tail of the Great Bear (Ursa Major). The meteor shower is named after the constellation of Quadrans Muralis (the mural quadrant - an archaic astronomical instrument which has long since been abandoned).

Jupiter and Saturn are well visible in the evening sky, over to the south- west. Jupiter is brighter than any of the stars, while Saturn - to its left - is dimmer and distinctly yellow.

Around midnight, reddish Mars rises in the east. At the moment, it's no more brilliant than many stars, but is getting brighter as Earth and Mars head for closest approach in April. Towards the end of January, you may spot the most brilliant planet of all, Venus, low in the west after sunset.

Heather Couper and Nigel Henbest