Galaxy Problems

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Earlier 12 Billion Years ago Later

A Dwarf Satellite Galaxy in Sagittarius

R A Ibata et al
July 21 1994
Nature

Galaxies

Half of Dark Matter in universe is White Dwarf Stars

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 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. 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."

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