Most of the stars in our Milky Way galaxy lie in a very flat, pinwheel-shaped disk. Although this disk is prominent in images of galaxies similar to the Milky Way, there is also a very diffuse spherical "halo" of stars surrounding and enclosing the disks of such galaxies.

Recent discoveries have shown that this outer halo of the Milky Way is probably composed of small companion galaxies ripped to shreds as they orbited the Milky Way.

A discovery announced by the Sloan Digital Sky Survey (SDSS) reveals a clump of stars unlike any seen before. The findings may shed light on how the Milky Way's stellar halo formed.

This clump of newly discovered stars, called SDSSJ1049+5103 or Willman 1, is so faint that it could only be found as a slight increase in the number of faint stars in a small region of the sky.

"We discovered this object in a search for extremely dim companion galaxies to the Milky Way," explains Beth Willman of New York University's Center for Cosmology and Particle Physics. "However, it is 200 times less luminous than any galaxy previously seen."

Another possibility, adds Michael Blanton, an SDSS colleague of Willman's at New York University, is that Willman 1 is an unusual type of globular cluster, a spherical agglomeration of thousands to millions of old stars."

"Its properties are rather unusual for a globular cluster. It is dimmer than all but three known globular clusters. Moreover, these dim globular clusters are all much more compact than Willman 1", explains Blanton. "If it's a globular cluster, it is probably being torn to shreds by the gravitational tides of the Milky Way."

The real distinction between the globular cluster and dwarf galaxy interpretations is that galaxies are usually accompanied by substantial quantities of dark matter, says Julianne Dalcanton, an SDSS researcher at the University of Washington. "Clearly the next step is to carry out additional measurements to determine whether there is any dark matter associated with Willman 1."

SDSS consortium member Daniel Zucker of the Max Planck Institute for Astronomy in Heidelberg, Germany, says the Sloan Digital Sky Survey has proven to be "a veritable gold mine for studies of the outer parts of our galaxy and its neighbors, as shown by Dr. Willman's discovery, and by our group's earlier discovery of a giant stellar structure and a new satellite galaxy around the Andromeda Galaxy."

If Willman 1 does turn out to be a dwarf galaxy, this discovery could shed light on a long-standing mystery.

The prevailing 'Cold Dark Matter' model predicts that our own Milky Way galaxy is surrounded by hundreds of dark matter clumps, each a few hundred light years in size and possibly populated by a dwarf galaxy.

However, only 11 dwarf galaxies have been discovered orbiting the Milky Way. Perhaps some of these clumps have very few embedded stars, making the galaxies particularly difficult to find.

"If this new object is in fact a dwarf galaxy, it may be the tip of the iceberg of a yet unseen population of ultra-faint dwarf galaxies," suggests Willman.

The Milky Way has been an area of intense research by SDSS consortium members.

"The colors of the stars in Willman 1 are similar to those in the Sagittarius tidal stream, a former dwarf companion galaxy to the Milky Way now in the process of merging into the main body of our Galaxy," explains Brian Yanny, an SDSS astrophysicist at The Department of Energy's Fermi National Accelerator Laboratory, a leader in research on the Milky Way's accretion of material.

Continues Yanny: "If Willman 1 is a globular cluster, then it may have piggybacked a ride into our Galaxy's neighborhood on one of these dwarf companions, like a tiny mite riding in on a flea as it, in turn, latches onto a massive dog."

"Whether it is a globular cluster or a dwarf galaxy, this very faint object appears to represent one of the building blocks of the Milky Way," Willman said.

The Sloan Digital Sky Survey (www.sdss.org) is the most ambitious survey of the sky ever undertaken. With more than 200 astronomers in 13 institutions around the world, the SDSS is making a map of one quarter of the entire sky, determining the position and brightness of hundreds of millions of celestial objects. It will also measure the distances to more than a million galaxies and quasars from the Apache Point Observatory in New Mexico.

The Astrophysical Research Consortium (ARC) manages the SDSS for the Participating Institutions. The Participating Institutions are The University of Chicago, The U.S. Department of Energy's Fermi National Accelerator Laboratory, The Institute for Advanced Study, The Japan Participation Group, The Johns Hopkins University, The Korean Scientist Group, Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, University of Pittsburgh, Princeton University, the United States Naval Observatory and the University of Washington.

Funding for the project has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society.