FOR RELEASE: June 1, 1999

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VLBA Detects Earth's Motion Around the Milky Way's Center

It takes our Solar System more than 200 million years to orbit the center of the Milky Way Galaxy, 26,000 light-years away. Despite that tremendously long time span, astronomers using the National Science Foundation's (NSF) powerful Very Long Baseline Array (VLBA) radio telescope have shown they can detect this orbital motion in ten days! In addition, they have made a new and more accurate determination of just how long it takes us to circle our Galaxy -- 226 million years.

"Not only is this a tremendous technical achievement, but it also has allowed us to greatly strengthen the scientific case for a supermassive black hole at the Galaxy's center -- definitely ruling out a multiple-star system," said Mark Reid of the Harvard-Smithsonian Center for Astrophysics. Reid, along with Anthony Readhead and Rene Vermuelen of Caltech and Robert Treuhaft of the Jet Propulsion Laboratory, announced their discovery at the American Astronomical Society's meeting in Chicago.

The scientists used the continent-wide VLBA, part of the NSF's National Radio Astronomy Observatory (NRAO), to observe a radio-wave-emitting object called Sagittarius A* (pronounced "A-star"), that has been thought to mark the exact center of the Milky Way since its discovery more than two decades ago. They were able to measure its position on the sky within nearly one ten-thousandth of a second of arc -- a precision 600,000 times greater than that of the human eye and more than 500 times greater than that of the Hubble Space Telescope.

With this precision, the astronomers were able to detect the slight apparent shift in position of Sagittarius A* compared to the positions of much more-distant quasars behind it. That apparent shift was caused by the motion of the Solar System around the Galaxy's center. "From these measurements, we estimate that we are moving at about 135 miles per second in our orbit around the center of the Milky Way," Reid said. "Even though it takes more than 200 million years for us to complete an orbit of the Galaxy's center, we can detect this motion in ten days' observing with the VLBA!"

The observations allowed the scientists to calculate a more accurate figure for how long it takes the Solar System to orbit the Galaxy's center. "Our new figure of 226 million years is accurate to within six percent," Reid said.

"The measurements we made with the VLBA place Sagittarius A* very close to, and most likely at, the exact (dynamical) center of our Galaxy, just as we expected," Reid said. "Right now, the new data also indicate that the minimum mass for this object is about 1,000 times the mass of the Sun. This rules out a multiple-star system and strengthens the idea that this object, much smaller than our own Solar System, contains a black hole about 2.6 million times more massive than the Sun," Reid added.

"We plan future observations that will increase the accuracy of our measurements even further, and, we suspect, raise the minimum mass for this object by as much as 100 times," Reid said.

The Milky Way's center, a complex region containing not only Sagittarius A* but also numerous supernova remnants and magnetic features, is obscured from optical telescopes by dust. Sagittarius A* was discovered in 1974 by astronomers using radio telescopes at the NRAO facility in Green Bank, WV. The NRAO's Very Large Array (VLA) and the VLBA both have been used for numerous studies of the Galactic Center region.

The VLBA is a system of ten radio-telescope antennas, each 25 meters (82 feet) in diameter, stretching some 5,000 miles from Mauna Kea in Hawaii to St. Croix in the U.S. Virgin Islands. Operated from NRAO's Array Operations Center in Socorro, NM, the VLBA offers astronomers the greatest resolving power, or ability to see fine detail, of any telescope currently operational. The NRAO is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

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The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.