The existence of such a population could partially explain the enduring mystery of the nature of dark matter in our Galaxy. The discovery will be announced in Nature on 6th January 2000 in a paper entitled 'Infrared spectrum of an extremely cool white dwarf'.

The object, named WD0346+246, was serendipitously discovered by Dr. Nigel Hambly (Edinburgh) as a faint, very fast moving star on a sequence of photographic plates. These plates, taken with the UK Schmidt Telescope in Australia, showed that the object was traversing the sky at 1.3 arcseconds per year, which is much faster than the majority of the Sun's neighbours.

This high apparent velocity is a characteristic of stars which are very old and are traveling on inclined elliptical orbits around the Galaxy. Most stars, like the Sun, proceed around the Galaxy's centre on a circular orbit.

A White Dwarf is the fossil remnant of a collapsed star which has reached the end of its normal life. Most stars will end their lives as white dwarfs, slowly cooling and fading away, like a dying ember. Thus the coolest white dwarfs are also the oldest stars and can help us to measure the age of the Galaxy.

A team, led by Drs. Hambly and Simon Hodgkin (Leicester/Cambridge), secured parallax measurements on the Jacobus Kapteyn Telescope (on the island of La Palma) to determine the distance to WD0346+246 and confirm its low luminosity.

They report a distance of 28 parsecs (about 90 light years). They also used the larger William Herschel Telescope (La Palma), the United Kingdom Infra Red Telescope (Hawaii) and the Keck Telescope (Hawaii) to look in detail at the star's visible colours, and its infrared emission. Recent theory has argued that such cool white dwarfs should contain molecular hydrogen in their outer layers which partially blocks infrared radiation trying to escape from the star.

This infrared emission is beyond what the eye can see, but the observations by Hodgkin et al. using precise, cooled instruments on large aperture telescopes shows a huge drop in the amount of infrared radiation emitted by WD0346+246 and confirms that the theory is correct. They estimate a surface temperature of around 3500 Kelvin for the white dwarf.

Astronomers have been searching for very cool white dwarfs for many years but with little success. This now appears to be partly because they were looking for objects which were red. WD0346+246 is in fact very blue in the infrared, and supports the claims from the theoreticians, that observers have simply been looking for the wrong kind of objects.

This discovery has serious implications for our understanding of the Milky Way. The coolest white dwarfs provide a measurement of the age of the Galaxy. But they may also play a more important role. For the last thirty years, astronomers have found that most of our Galaxy seems to be invisible. In fact, as much as 90% of the mass in our Galaxy may be hidden in the form of 'dark matter'.

Dark matter theories fall into two broad classes. The first suggests that the dark matter is not really dark -- but is composed of many faint stars such as cool white dwarfs and brown dwarfs. The second class of dark matter candidates are various elementary particles, left over from the big bang.

Indirect evidence for the 'dark matter' being comprised of cool white dwarfs first came from the MACHO (MAssive Compact Halo Object) gravitational microlensing experiment. The MACHO project monitored some ten million stars in the Magellanic Clouds in the hope of detecting the occasional brightening caused by a dark Halo object moving across our line of sight to one of the stars.

An intervening object can act as a gravitational lens, focusing light and causing a temporary increase in the observed brightness of a background star. By measuring the duration of the brightening, astronomers get a crude measure of the mass of the otherwise invisible forgeround object.

Microlensing experiments suggested the existence of a large number of dark objects in the Halo of our Galaxy with masses about half that of our Sun. The likely candidates for these invisible objects are distant, faint, cool white dwarfs.

However as such objects had never actually been seen before there was some doubt as to their nature. The MACHO results suggest that these stars are very numerous, and could contribute approximately 50% of the total mass of the Galaxy.

The discovery of one nearby, very old and cool white dwarf does not solve the dark matter problem. But it does lend weight to the MACHO scenario, and presents astronomers with an astonishing conclusion: the Galaxy may be full of extremely old white dwarf stars. The race is now on to count how many objects like WD0346+246 exist in the Galaxy and to measure how much they weigh in total.