Jan. 9, 2001 SAN DIEGO, Calif. (Jan. 8, 2001) -- Astronomers have found 154 rapidly moving stars towards the center of our galaxy and our brightest neighboring galaxy. The findings are being presented today by Dr. Andrew J. Drake of the Lawrence Livermore National Laboratory for the Massive Compact Halo Objects (MACHO) collaboration, during the annual meeting of the American Astronomical Society in San Diego, Calif. The results are of special interest because this is the first time scientists have been able to discover such objects in front of the millions of stars seen at the Galactic center and our brightest neighbor galaxy, the Large Magellanic Cloud (LMC).
To date, among the thousands of known High Proper Motion (HPM) stars, few have been discovered in the most densely packed regions of the sky, where stars appear to merge together in images because of their extreme density.
``Until now astronomers have been unable to detect HPM stars in the most dense locations because of the extreme density of stars towards the Galactic center,'' said Drake, who works at Livermore’s Institute for Geophysics and Planetary Physics. "Toward the Galactic center, the billions of stars within our galaxy form the bright band in the sky known as the Milky Way."
Another region where the density of stars makes discovery of the moving ones difficult is towards the LMC. To the naked eye this galaxy appears as a faint nebulous patch in the southern sky. Through a small telescope the presence of millions of individual stars becomes recognizable.
Our solar system resides 26,000 light years from the center of the galaxy and rotates once every 240 million years. The great distance to the Galactic center means that the slow rotation of the Sun has little effect on stars there. However, much closer stars (less than 500 light years) appear to move relative to these distant stars. In order to find HPM stars, Drake looked at images of stars in the Galactic center and the LMC taken over seven years.
Using 50 thousand astronomical images of 55 million stars, Drake identified the stars that appear to move and measured their motions. From these measurements, he discovered 154 new HPM stars. The yearly motions of these objects are estimated to be accurate to 6 milli-arcseconds, which is equivalent to the width of a human hair seen from a distance of a mile.
These images came from a recent galactic dark matter experiment using the 50-inch Great Melbourne Telescope in Canberra, Australia. During the 1990s, scientists also used the Great Melbourne Telescope to detect MACHOs (Massive Compact Halo Objects) through the gravitational microlensing of stars. Microlensing is a physical phenomenon which causes starts to appear to shift of brighten when two or more of them lie on the same line of sight.
Over the years, techniques such as astrometry have allowed astronomers to produce a picture of the motions of stars within our galaxy. Astrometry is the branch of astronomy that deals with the measurement of positions and movements. Applying this picture to the motions of the HPM stars discovered, Drake was able to predict that most of these objects likely are located at distances between 100 and 1000 light years.
However, at present, the motions of these newly discovered HPM stars have been based on the motion measured between just two images. More detailed studies of these stars are necessary to determine if they are very nearby, how the parallax effect, due to the Earth’s motion around the Sun, would change the true direction of each HPM star’s motion from that observed.
Although many microlensing events have been discovered, astronomers continue to search for them because they can point out properties of the lensing objects, such as planets, which populate our galaxy. Within the next ten years, NASA's Space Interferometry Mission (SIM) telescope will be launched into orbit. One of the goals of this mission is to use astrometry to determine the masses and distances of the stars causing microlensing events. By finding the HPM stars in the foreground of these dense areas of the sky and predicting their paths over future years, astronomers will be able determine when these stars will pass in front of a distant star to cause microlensing.
The MACHO collaboration is made up of: K.H. Cook, A.J. Drake, S.L. Marshall, C.A. Nelson and P.Popowski of the Lawrence Livermore National Laboratory; C. Alcock and M.J. Lehner from the University of Pennsylvania; R.A. Allsman of the Australian National Supercomputing Facility; D.R. Alves of STScI; T.S. Axelrod, K.C. Freeman and B.A. Peterson of the Mount Stromlo Observatory; A.C. Becker of Bell Labs; D.P. Bennett of the University of Notre Dame; M. Geha of University of California at Santa Cruz; K. Griest and T. Vandehei of the University of California at San Diego; D. Minniti of Universidad Catolica; M.R. Pratt, C.W. Stubbs and A.B. Tomaney of the University of Washington; P.J. Quinn of the European Southern Observatory; W. Sutherland of the University of Oxford; and D. Welch of McMaster University.
Founded in 1952, Lawrence Livermore National Laboratory is a national security laboratory, with a mission to ensure national security and apply science and technology to the important issues of our time. The Lawrence Livermore National Laboratory is managed by the University of California for the U.S. Department of Energy.
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