A project supported by the National Science Foundation (NSF) to determine the nature of dark matter in the halo of the Milky Way has yielded a treasure trove of data on 73 million stars. Many of them are variable stars, whose brightness varies over time and which are particularly useful for astronomical research. The database, created by an international team in Australia and the United States, has been made available to astronomers worldwide via the World Wide Web.

The Massive Compact Halo Objects (MACHO) team scrutinized the Large and Small Magellanic Clouds, two galaxies which orbit the Milky Way, and the bulge of the Milky Way in an eight-year search for massive objects such as planets or brown dwarfs. These objects are believed to make up much of the dark matter in the region. They can be detected through gravitational lensing, in which the light reaching Earth from extragalactic stars is amplified due to the gravitational force generated by nearby massive objects.

As a byproduct, the search yielded images and light curves of 73 million stars. The brightness of many of these stars varies in a regular pattern, and their light curves chart the pattern.

“The light curve is a window into the heart of a star, providing us with information that is not available in any other way,” said Morris Aizenman, a senior science advisor at NSF. “As these data are analyzed by the worldís scientific community, they are certain to reveal some surprises.” One type of variable stars, Cepheid variables, are useful as “meter sticks” for measuring distances in the universe. The data could also be useful for studying the interiors of stars and their evolution and estimating the age of the universe.

The light amplifications sought by the MACHO scientists are so rare that, in order to generate useful data, they examined millions of stars, using the 1.3-meter Great Melbourne Telescope at Mt. Stromlo Observatory, Australia. They believe they have found as many as 40 potential candidates for massive objects.

The images, light curves and a catalog of the variable stars are available for viewing or downloading from the MACHO project websites at http://www.macho.mcmaster.ca and
http://www.macho.anu.edu.au. Sophisticated search engines and image analysis tools assist researchers accessing the data.

“The combination of large databases and computational tools are speeding scientific discovery in all fields, and we wanted to expand this capability for astronomers,” said U.S. team leader Charles Alcock of the University of Pennsylvania. Alcock started the MACHO project in 1990 along with Tim Axelrod, formerly of Lawrence Livermore Lab and now of the Australian National University. Team members Kem Cook of Livermore and Robyn Allsman of the Australian National University led the effort to make the data available.

The MACHO project received support from the NSF-supported Center for Particle Astrophysics at the University of California at Berkeley, Santa Barbara and San Diego; the Department of Energyís Lawrence Livermore National Laboratory; and the Australian National University.