Hubble Rules Out A Theory Of Dark Matter

Ground-Based and Shuttle-Based Images of A Globular Cluster

Two teams of astronomers, working independently with NASA's Hubble Space Telescope (HST) have ruled out the possibility that red dwarf stars constitute the invisible matter called "dark matter", believed to account for more than 90 percent of the mass of the universe.

Until now, the dim, small stars were considered ideal candidates for dark matter. Whatever dark matter is, its gravitational pull ultimately will determine whether the universe will expand forever or will someday collapse. "Our results increase the mystery of the missing mass. They rule out a popular but conservative interpretation of dark matter," said Dr. John Bahcall at the Institute of Advanced Study, Princeton, NJ, and leader of one of the teams. Dr. Bahcall and Andrew Gould of Ohio State University, showed that faint red dwarf stars, which were thought to be abundant, actually are sparse in the Earth's home galaxy, and in the universe by inference.

The team, led by Dr. Francesco Paresce of the Space Telescope Science Institute in Baltimore, MD, and the European Space Agency, determined that the faint red stars rarely form and that there is a cutoff point below which nature does not make this type of low-mass star.

The space telescope observations involved accurately counting stars and gauging their brightness. The observations overturn several decades of conjecture about the typical mass and abundance of the smallest stars in the universe. In our own stellar neighborhood, there are almost as many red dwarfs as there are all other types of stars put together. The general trend throughout our galaxy is that small stars are more plentiful than larger stars, just as there are more pebbles on the beach than rocks. This led many astronomers to believe that they were only seeing the tip of the iceberg and that many more extremely faint red dwarf stars were at the limits of detection with ground-based instruments.

According to stellar evolution theory, stars as small as eight percent of the mass of the Sun are still capable of shining by nuclear fusion processes. Over the past two decades, theoreticians have suggested that the lowest mass stars also should be the most prevalent and therefore might provide a solution for dark matter. This seemed to be supported by previous observations with ground-based telescopes that hinted at an unexpected abundance of what appeared to be red stars at the faintest detection levels achievable from the ground. However, these prior observations were uncertain because the light from these faint objects is blurred slightly by Earth's turbulent atmosphere. This makes the red stars appear indistinguishable from the far more distant, diffuse-looking galaxies.

Hubble's capabilities made it possible to observe red stars that are 100 times dimmer than those detectable from the ground -- a level where stars can be distinguished easily from galaxies. Hubble's extremely high resolution also can separate faint stars from the much more numerous galaxies by resolving the stars as distinct points of light, as opposed to the "fuzzy" extended signature of a remote galaxy.

Bahcall and Gould used images of random areas in the sky taken with the HST Wide Field Planetary Camera 2 while the telescope was performing scheduled observations with other instruments. By simply counting the number of faint stars in the areas observed by HST, the scientists demonstrated that the Milky Way has relatively few faint red stars.

The HST observations show that dim red stars make up no more than six percent of the mass in the halo of the Galaxy, and no more than 15 percent of the mass of the Milky Way's disk.

By coincidence, Paresce pursued the search for faint red dwarfs after his curiosity was piqued by an HST image taken near the core of the globular cluster NGC 6397. He was surprised to see that the inner region was so devoid of stars, he could see right through the cluster to far more distant background galaxies. Computer models of stellar population predicted the field should be saturated with dim stars-- but it wasn't.

HST's resolution allowed the most complete study to date of the population of the cluster (globular clusters are pristine laboratories for studying stellar evolution). To Paresce's surprise, he found that stars 1/5 the mass of our Sun are very abundant -- there are about 100 stars this size for every single star the mass of our Sun -- but that stars below that range are rare. "The very small stars simply don't exist," he said.

A star is born as a result of the collapse of a cloud of interstellar gas and dust. This contraction stops when the infalling gas is hot and dense enough to trigger nuclear fusion, causing the star to radiate energy. "There must be a mass limit below which the material is unstable and cannot make stars," Paresce emphasizes. "Apparently, nature breaks things off below this threshold."