SETI [ASTRO] VLBA Detects Earth's Motion Around the Milky Way's Center

Larry Klaes (
Mon, 07 Jun 1999 21:00:11 -0400

>X-Authentication-Warning: majordom set sender to owner-astro using -f >Date: Mon, 7 Jun 1999 19:20:06 GMT >From: Ron Baalke <> >To: >Subject: [ASTRO] VLBA Detects Earth's Motion Around the Milky Way's Center >Sender: >Reply-To: Ron Baalke <> > >National Radio Astronomy Observatory >P.O. Box O >Socorro, New Mexico 87801 > > >Contact: >Dave Finley, Public Information Officer >(505) 835-7302 > >FOR RELEASE: June 1, 1999 > >VLBA Detects Earth's Motion Around the Milky Way's Center > >Our Solar System orbits the center of the Milky Way Galaxy, 26,000 >light-years away, only once every 200 million years. 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 less than a month! > >"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 200 >million years for us to complete an orbit of the Galaxy's center, we can >detect this motion in less than a month's observing with the VLBA!" > >"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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