A brilliant comet that dazzled observers over Easter in 1997 is continuing to astonish astronomers 14 years later. Hale-Bopp, which shone brightly with a spectacular tail for weeks on end, is still showing signs of activity despite having receded to the edge of the solar system.

The comet now lies far beyond the orbit of the most distant planet Neptune and becomes the furthest comet from the Sun ever observed, smashing a record set by Halley. But recent observations indicate that it is still spewing gases, long after most comets would have become frozen, dead balls of ice and rock once more.

A team of Hungarian astronomers used the European Southern Observatory’s 2.2m telescope at La Silla, Chile, to observe Hale-Bopp at a distance of more then 28.5 billion miles (46 billion km) – further than a comet has ever been observed before. Though it is now so far and faint, they found that its brightness was greater than should be expected at such a distance.

This brightness is greater than could be explained merely by sunlight reflecting from the comet’s icy surface. The astronomers, led by Gyula Szabo, conclude that this means it means it still surrounded by a faint shell of gas, called a coma – a temporary atmosphere produced by evaporating gases.

They compare Hale-Bopp’s behaviour to that of another famous comet, Halley, which was previously the most distant one to be observed. Halley’s activity ceased at a distance of around 10 billion miles (16 billion km), following its last appearance in 1986, though it did display a later outburst in 1991 at a distance of 13 billion miles (21 billion km), briefly flaring in brightness by five magnitudes.

Comets are lumps of ice and rock, usually no more than a few miles wide, which have become dislodged from a vast reservoir of material surrounding the solar system, called the Oort Cloud. Spaceprobes have given us rare close-ups of them.

This frozen shell of debris is believed left over from the formation of the solar system more than four billion years ago and its pristine condition makes comets of great interest to astronomers.

The observations for the scientific paper, which will be published in the journal Astronomy & Astrophysics, were carried out on December 4, 2010.