Dead stars could be 'missing mass'

A white dwarf imaged in 1954 (l) and in 1994 (r)

By BBC News Online science editor Dr David Whitehouse

Astronomers have identified swarms of dead stars they think could form part of the mysterious and unseen component of the Universe referred to as dark matter.

Only a few percent of the mass of the cosmos can be directly observed. The rest - the "missing mass" - is detectable only because of its gravitational influence on the objects we can see.

Now, a detailed analysis of images of a selected region of the southern sky has revealed 38 previously unseen white-dwarf stars, believed to be part of an extensive halo of old stellar objects enveloping our Milky Way galaxy.

These cosmic cinders, although hot, are little bigger than the Earth and are therefore not easy to detect. But if, as now seems likely, these objects are spread evenly throughout the halo, they could account for 3-35% of the missing mass.

Exotic matter

Dark matter has puzzled astronomers since the discovery in 1933 that some unseen form of matter was dominating the structure and motion of galaxies.

Two possible dark matter candidates

Wimps (Weakly Interacting Massive Particles) - Exotic subatomic particles, such as axions, massive neutrinos, photinos

Machos (Massive Compact Halo Objects) - Neutron stars, black holes, white dwarfs, brown dwarfs

But scientists have struggled to determine the precise nature of this material. According to the latest theories, only 5%, at most, of the Universe is made up of "normal", so-called baryonic matter. This includes the visible stars and planets.

Some of the missing mass is probably normal, but for some reason cannot be picked up by our telescopes. And some of the mass is probably exotic, as yet undefined, material which interacts only weakly with normal matter, also making its detection very difficult.

What the new discovery, reported in the journal Science, now suggests is that old, burnt-out stars may actually account for at least some of the normal (or not-so-exotic) part of dark matter.

Nuclear fuel

The researchers scoured 30-year-old photographic plates of regions of the southern sky that had recently been computerised at the Royal Observatory, Edinburgh, UK.

Searching through about 10% of the data, the team found 92 suspicious objects.

Follow-up observations from the Cerro Tololo Observatory in Chile revealed that 38 of these targets were previously unseen white-dwarf stars within 450 light-years of Earth.

According to the scientists, what they have found may be just the tip of an astronomical iceberg. More than 99% of stars, including our Sun, end their lives as white dwarfs - they are hot, dark spheres that have exhausted their nuclear fuel.

And it is estimated that the stars detected in this new survey are ancient, at least 10 billion years old. There must be many, many more out there.

Star formation

Dr Andrew Digby, from the University of Edinburgh, one of the scientists involved in the study, told BBC News Online: "The dark matter problem has been with us for 70 years. This discovery could help us work out how the galaxy started in the first place."

Co-researcher Ben Oppenheimer, from the University of California at Berkeley, US, said: "We've found a previously undetected population of stars in the galactic halo that represents a fraction of the baryonic dark matter in the galaxy.

"This raises a lot of questions about our understanding of the star formation history of the galaxy and the basic processes of star formation."

And Didier Saumon, of Vanderbilt University, US, added: "These cool, white dwarfs are the fossils of the early population of halo stars. There is much to learn about how galaxies form, and about how stars form in the process, from studying these white dwarfs."