Search of Galactic Halo Yields a Treasure Trove of Variable Stars

A project
supported by the United States Department of Energy (DOE) and 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 variable.

This database, created by an international team in Australia and
the United States, has been made available to astronomy researchers
worldwide via the Internet.

The Massive Compact Halo Objects (MACHO) team scrutinized the
Large and Small Magellanic Clouds, two galaxies that 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, believed to make up
much of the dark matter there. These objects can be detected through
gravitational lensing, in which the light reaching Earth from the
extragalactic stars is magnified due to the gravitational force
generated by the 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.

“A particularly useful feature of this data release is that we
provide period, amplitude and tentative classification information in
a catalog for periodically varying stars in the Large Magellanic
Cloud,” said Kem Cook, of Lawrence Livermore National Laboratory, who
has led the variable star work for the project.

“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 adviser at NSF. “As these data are
analyzed by the world’s scientific community, they are certain to
reveal some surprises.”

The Cepheid variables, one type of variable star, are useful as
“meter sticks” for measuring distances in the universe. Other
potential uses of the data include 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 in more than 200 separate regions, using the 1.3-meter Great
Melbourne Telescope at Mt. Stromlo Observatory, Australia. They found
almost 20 potential candidates for massive objects in the halo of the
Milky Way in a partial analysis of their data.

The lightcurves, along with images 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://wwwmacho.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 at
Lawrence Livermore in 1990 along with Cook and Tim Axelrod, formerly
of Lawrence Livermore and now of the Australian National University.

Another team member, Robyn Allsman of the Australian National
University, took the lead in making the data available.

“The MACHO web delivery system grew from my determination that the
data should outlive the MACHO Project itself,” Allsman said. “Use of
emerging standards, such as GLU and XML, enabled the data to merge
into the evolving network of linked astronomical data catalogs, and
position it for inclusion in future virtual observatories.” The MACHO
project received support from the NSF-supported Center for Particle
Astrophysics at the Universities of California at Berkeley, Santa
Barbara and San Diego; DOE’s Lawrence Livermore National Laboratory;
and the Australian National University.

Founded in 1952, Lawrence Livermore National Laboratory is a
national nuclear security laboratory, with a mission to ensure
national security and apply science and technology to the important
issues of our time. The National Nuclear Security Administration’s
Lawrence Livermore National Laboratory is managed by the University of
California.

Laboratory news releases and photos are also available
electronically on the World Wide Web of the Internet at URL
http://www.llnl.gov/PAO and on UC Newswire.