Coolest star sheds light on dark matter

British astronomers have discovered the faintest, coolest star ever, with a mass only 50 times greater than that of the planet Jupiter. Despite being so faint that it required special techniques to detect it, the "brown dwarf" could help to solve the enduring puzzle of the universe's dark matter.

The star is known as a brown dwarf because it is small and comparatively cool. According to observations, its surface temperature is 2,000C. By contrast, our sun has a surface temperature of 5,500C, and a mass 1,000 times greater than Jupiter.

Dubbed PIZ 1 by the team of astronomers from the University of Leicester which discovered it, the star lies in the Pleiades star cluster. Its low mass suggests it has never become hot enough to become a self-fuelling star: the heat of the surface derives from the time when gravity pulled it together. The discovery was announced at this week's National Astronomy Meeting in Southampton.

Dark matter - which is impossible to detect by normal means - is believed to constitute more than 90 per cent of the total mass of the universe; the visible stars cannot explain the wider gravitational effects that astronomers observe. But theoreticians have argued over whether the dark matter consists of planet-sized chunks of real matter, or could be comprised of subatomic particles such as neutrinos which would make up for their tiny individual mass by weight of numbers.

"This discovery has implications for dark matter because low mass stars and objects could make up the unseen matter in the universe," said David Pinfield, one of the four-strong team which made the breakthrough. "It hasn't been clear before whether the form of dark matter goes all the way down to masses the size of Jupiter. Now we have shown that it does. Jupiter-sized objects would count as dark matter."

Further observations could help to establish the total mass of the Pleiades cluster, as the observed stars have in effect already been "weighed" by estimating their mass, based on their brightness.

After this year's stunning show from Hale-Bopp comet, November next year should see a spectacular meteor shower when the Earth passes through the dust grains of another comet.The result will produce bright streaks in the sky, with thousands of meteors per hour entering the Earth's atmosphere and burning up.

Though they will also be visible this November, next year is expected to bring a more impressive show.