Hubble Observations Deepen The Mystery of Dark Matter

Two teams of astronomers, working independently with the Hubble Space Telescope (HST), have ruled out the possibility that red dwarf stars constitute the so-called dark matter, believed to account for more than 90% of the mass of the universe. Until now, the dim, small stars were considered ideal candidates for dark matter by some astronomers.

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, of the Institute of Advanced Study, Princeton, a leader of one of the teams. The group led by Bahcall and Andrew Gould of the Ohio State University, showed that faint red dwarf stars, which were thought to be abundant, actually are sparse in the Milky Way, and, by inference, in the universe.

The team led by Dr. Francesco Paresce of the Space Telescope Science Institute 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 dim, low-mass star.

The pair of HST observations involved accurately counting stars and gauging their brightness. The observations overturn several decades of conjecture, theory, and observation 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 groundbased instruments.

According to stellar evolution theory, stars as small as 8% of the mass of our 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 so might provide a solution for dark matter. This seemed to be supported by previous observations with groundbased 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 observations were uncertain because the light from these faint objects is blurred slightly by Earth's turbulent atmosphere. This makes the red stars indistinguishable from the far more distant, diffuse-looking galaxies.


The spacebased Hubble made it possible for astronomers 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 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.

The HST observations show that dim red stars make up no more than 6% of the mass in the halo of the Galaxy and no more than 15% of the mass of the Milky Way's disk. The Galactic halo is a vast spherical region that envelopes the Milky Way's spiral disk of stars.


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 that he could see right through the cluster to far more distant background galaxies. Computer simulations based on models of stellar population predicted the field should be saturated with dim stars--but it wasn't.

Paresce, and co-investigators Guido De Marchi (STScI and the University of Firenze), and Martino Romaniello (University of Pisa) used HST to conduct the most complete study to date of the population of the cluster (globular clusters are ancient, pristine laboratories for studying stellar evolution). To his 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 of solar mass) 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 gravitational 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 glow and 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." Paresce has considered the possibility that very low-mass stars formed long ago but were thrown out of the cluster as a result of interactions with more massive stars within the cluster or during passage through the plane of our Galaxy. This process would presumably be common among the approximately 150 globular clusters that orbit the Milky Way. However the cast-off stars would be expected to be found in the Milky Way's halo, and Bahcall's HST results don't support this explanation.


The findings are the latest in the astronomical quest to understand the puzzle of the universe's missing mass. Models of the origin of helium and other light elements during the Big Bang predict that less than 5% of the universe is made up of "normal stuff," such as neutrons and protons. This means more than 90% of the universe must be some unknown material that does not emit radiation that can be detected by current instruments. Candidates for dark matter include black holes, neutron stars, and a variety of exotic elementary particles.

Within the past year, astronomers have uncovered indirect evidence for a dark matter candidate called a MACHO (Massive Compact Halo Objects). These observations detected several instances of an invisible object that happens to lie along the line of sight to an extragalactic star. When the intervening object is briefly aligned between Earth and a distant star, it amplifies, or gravitationally lenses, the light from the distant star. The new HST finding shows that faint red stars are not abundant enough to explain the gravitational lensing events attributed to MACHOs. Bahcall cautions, however, that his results do not rule out other halo objects that could be smaller than the red stars such as brown dwarfs--objects not massive enough to burn hydrogen and shine in visible light.

Additional circumstantial evidence for dark matter in the halo of our galaxy has been inferred from its gravitational influence on the motions of stars within the Milky Way's disk. Recently, this notion was further supported by groundbased observations, made by Penny Sackett of the Institute for Advanced Study, that show a faint glow of light around a neighboring spiral galaxy that is the shape expected for a halo composed of dark matter. This could either be light from the dark matter itself or stars that trace the presence of the galaxy's dark matter.

The ultimate fate of the universe will be determined by the amount of dark matter present. Astronomers have calculated that the amount of matter--planets, stars, and galaxies--observed in the universe cannot exert enough gravitational pull to stop the expansion that began with the Big Bang. Therefore, if the universe contains less than a critical density of matter it will continue expanding forever, but if enough of the mysterious dark matter exists, the combined gravitational pull someday will cause the universe to stop expanding and eventually collapse.

A NASA Hubble Space Telescope image of a small region (1.4 light-years across) in the globular star cluster NGC 6397 shows far fewer stars than would be expected if faint red dwarf stars were abundant. HST resolves about 200 stars. The stellar density is so low that HST can literally see right through the cluster and resolve far more distant background galaxies. This observation shows the surprising cutoff point below which nature apparently doesn't make many stars smaller than 1/5 the mass of the Sun. If there were lower mass stars in the cluster, then the image would contain an estimated 500 stars.

Credit: F. Paresce, STScI & ESA and NASA.