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Monday, 10 December, 2001, 10:57 GMT
Hubble's dark conundrum
Red dwarf, Stsci
The red star is much closer than the blue one
By BBC News Online science editor Dr David Whitehouse

The Hubble Space Telescope (HST) may have solved one of the greatest problems in astronomy - the nature of the Universe's "missing mass" - or it may have found just another commonplace star.

The HST found a faint red dwarf star nestling close to a distant star whose light output flared a few years ago. Astronomers believe that the gravity of the dim, dwarf star acted as a "gravitational lens" that amplified the light from the more distant object behind.

Because most of the mass of the Universe is unseen - its presence is inferred from its gravity - the observation could be proof that dim, red dwarf stars - the most plentiful type of star in the cosmos - comprise a large chunk of the mysterious missing mass.

Alternatively, it could be that a common star just happened to be seen to pass in front of a distant one - an event that's bound to happen some time. Astronomers are uncertain which explanation to believe.

Andrew Gould, of Ohio State University, US, believes it is a breakthrough saying it opens up "a whole new era of astronomy".

Exotic or normal

The nature of dark matter is one of the greatest puzzles in astrophysics.

LMC, Stsci
The Large Magellanic Cloud is a close companion of the Milky Way
The motions of stars and galaxies suggest they are moving under the gravitational influence of matter that radiates no detectable light. Calculations suggest that most of the mass of the Universe falls into this category.

Astronomers are divided into two camps about its nature.

Some believe it consists of exotic, as yet unidentified, sub-atomic particles. Others contend it is in the form of "normal" matter - small, dense objects such as dim stars, black holes or planet-sized lumps of rock and ice.

For almost a decade, an international team of astronomers has been studying the Large Magellanic Cloud (LMC), a close companion of our Milky Way galaxy, to try to solve the mystery.

Dwarf star

The team, known as the Massive Compact Halo Objects (Macho) collaboration, has been looking for examples of starlight being amplified by the lensing effect of the gravity of an intervening object.

It has documented several such events in recent years, but this is the first time the object responsible for the lensing has been pictured directly.

The HST looked at a distant star whose light had been boosted some six years ago. Very close to the distant star, HST saw a small, faint, red dwarf star at a distance of 600 light-years from us. Its mass is between 5% and 10% that of our own Sun.

The result may have implications about the number of Machos in our galaxy, the Milky Way.

Some researchers think most Machos are to be found in a spherical halo that envelops our Milky Way.

Good method

However, the new star is not distant enough to be classed as being in the halo and is one member of a vast number of similar stars that swarm through our Milky Way. Because of this, it is not certain that the dim star can be identified with the Universe's missing mass.

However, the research, detailed in the latest edition of the journal Nature, does demonstrate a method for tracking down those objects that really are.

"We've proved the principle," said Dr Kem Cook, from the Lawrence Livermore National Laboratory, US, and one of the lead researchers on the project.

"If we see a few more of these towards the LMC, we'll be able to pin down where the lenses really are," he told BBC News Online. "If they're out in the halo, then its halo dark matter; if they're close into the galaxy, then we may have to re-think how the galaxy is built."

See also:

11 May 00 | Sci/Tech
Deflected light 'sees' dark matter
09 Mar 00 | Sci/Tech
New light on dark matter
14 Jan 00 | Sci/Tech
Lone drifter black holes discovered
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