Andrew Yee[_1_]
January 9th 08, 04:03 PM
National Radio Astronomy Observatory
P.O. Box O
Socorro, NM 87801
http://www.nrao.edu
Contact:
Dave Finley, Public Information Officer
(505) 835-7302
EMBARGOED For Release: 8:30 a.m., CST, Wednesday, January 9, 2008
Radio Telescopes' Precise Measurements Yield Rich Scientific Payoffs
Having the sharpest pictures always is a big advantage, and a sophisticated
radio-astronomy technique using continent-wide and even intercontinental
arrays of telescopes is yielding extremely valuable scientific results in a
wide range of specialties. That's the message delivered to the American
Astronomical Society's meeting in Austin, Texas, by Mark Reid of the
Harvard-Smithsonian Center for Astrophysics, a leading researcher in the
field of ultra-precise astronomical position measurements.
"Using radio telescopes, we are measuring distances and motions of celestial
bodies with unprecedented accuracy. That's helping us better understand many
processes ranging from star formation to the scale of the entire Universe,"
Reid said.
The observing technique, called Very Long Baseline Interferometry (VLBI),
was pioneered in 1967, but has come into continuous use only in the past
10-15 years. The National Science Foundation's Very Long Baseline Array
(VLBA), a system of 10 radio-telescope antennas ranging from Hawaii to the
Caribbean, was dedicated in 1993. There are other VLBI systems in Europe and
Asia, and large radio telescopes around the world cooperate regularly to
increase sensitivity. VLBI observations routinely produce images hundreds of
times more detailed than those made at visible-light wavelengths by the
Hubble Space Telescope.
Several groups of researchers from across the globe use the VLBA to study
stellar nurseries in our own Milky Way Galaxy and measure distances to
regions where new stars are forming. The key has been to improve measurement
accuracy to a factor of a hundred times better than that produced by the
highly successful Hipparcos satellite. Using small clouds of gas in
star-forming regions that strongly amplify radio waves, called cosmic
masers, the astronomers measured the tiny shift in the object's position in
the sky caused by the Earth's orbit around the sun. This, in turn, yielded
highly-accurate distances by the simple surveying technique of
triangulation, the "gold standard" of distance measuring techniques
available to astronomers.
"Knowing the distance accurately means we also know the luminosities, masses
and ages of the young stars much more accurately, and that is vital to
understanding how star formation works," Reid said. In addition, he pointed
out, the VLBA observations have shown the motions of the young stars in the
Milky Way are much more complicated than simple circular motion. Massive
young stars appear to be born orbiting the Milky Way considerably slower
than older stars. "This might be explained by the interaction of giant
molecular clouds, the ultimate sites of massive star formation, as they
"surf" spiral density waves in the Milky Way."
An international team of scientists led by Reid has used VLBI to detect the
slight change in apparent position of the object at the Milky Way's center
caused by our Solar System's orbit around that center. "It takes our Solar
System more than 200 million years to circle the center of our Galaxy, and
yet we can detect that motion in only a couple weeks with the VLBA -- truly
astounding!" Reid said.
The VLBA studies of the Galactic Center have shown that an object called
Sagittarius A* is at the exact gravitational center of our Galaxy. That
means, the scientists say, that the object must be incredibly massive. "The
VLBA measurements, combined with infrared observations of stellar orbits
around this object, provide overwhelming evidence that it's a supermassive
black hole," Reid explained. "These observations are also going to make it
possible to re-define the coordinate system used to map the entire Galaxy,"
Reid added.
Looking farther outward, astronomers achieved a longstanding goal of
measuring the spin of another galaxy. In 2005, Reid and his colleagues
measured both the rotational spin and the motion in space of the galaxy M33,
nearly 2.4 million light-years from Earth. Astronomers in the 1920s had
attempted such a feat, but their results were not accurate enough. "This
achievement had to wait for the VLBA," Reid said. This and subsequent work
has put strong limits on the amount of unseen "dark matter" around the giant
Andromeda galaxy, which M33 orbits. A continuing goal is to use VLBI
observations to measure the orbits of these and other galaxies within the
Local Group of galaxies to which our own Milky Way belongs.
In 1999, astronomers set a new standard for a distance measurement outside
the Local Group of galaxies when they used the VLBA to make a direct
geometric distance measurement to a galaxy called NGC 4258, 23.5 million
light-years from Earth. That measurement, accurate to within 7 percent,
caused other scientists to revise their indirect-measurement techniques for
the rest of the Universe. The NGC 4258 distance was calculated by measuring
the motion of masers in a disk of gas containing water molecules and
orbiting a supermassive black hole at the galaxy's center.
"Now, other galaxies are being observed in hopes of extending direct
distance measurement even farther out in the Universe," Reid said. "One
candidate, called UGC 3789, at a distance of about 160 million light-years,
will be measured with about 10 percent accuracy. Our goal is to further
improve these measurements and to measure 5 to 10 other galaxies in order to
determine the Hubble constant (the expansion rate of the Universe) to 3
percent accuracy. This would put limits on key parameters of the dark energy
that apparently is accelerating the expansion of the Universe," Reid added.
The kind of accurate measurement of distances and motions that VLBI
observations provide can benefit numerous other areas of astronomy, Reid
pointed out. For example, the distances to pulsars have been measured
directly with the VLBA, yielding better understanding of their
characteristics. The technique also could reveal planets circling some
nearby stars.
"Anytime you can do something as dramatic as improving measurement accuracy
by a hundredfold, you're bound to get a great scientific payoff," Reid said.
"We're looking forward to exciting new results in the coming years," he
added.
The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.
[NOTE: Images supporting this release are available at
http://www.nrao.edu/pr/2008/vlbiastrometry/ ]
P.O. Box O
Socorro, NM 87801
http://www.nrao.edu
Contact:
Dave Finley, Public Information Officer
(505) 835-7302
EMBARGOED For Release: 8:30 a.m., CST, Wednesday, January 9, 2008
Radio Telescopes' Precise Measurements Yield Rich Scientific Payoffs
Having the sharpest pictures always is a big advantage, and a sophisticated
radio-astronomy technique using continent-wide and even intercontinental
arrays of telescopes is yielding extremely valuable scientific results in a
wide range of specialties. That's the message delivered to the American
Astronomical Society's meeting in Austin, Texas, by Mark Reid of the
Harvard-Smithsonian Center for Astrophysics, a leading researcher in the
field of ultra-precise astronomical position measurements.
"Using radio telescopes, we are measuring distances and motions of celestial
bodies with unprecedented accuracy. That's helping us better understand many
processes ranging from star formation to the scale of the entire Universe,"
Reid said.
The observing technique, called Very Long Baseline Interferometry (VLBI),
was pioneered in 1967, but has come into continuous use only in the past
10-15 years. The National Science Foundation's Very Long Baseline Array
(VLBA), a system of 10 radio-telescope antennas ranging from Hawaii to the
Caribbean, was dedicated in 1993. There are other VLBI systems in Europe and
Asia, and large radio telescopes around the world cooperate regularly to
increase sensitivity. VLBI observations routinely produce images hundreds of
times more detailed than those made at visible-light wavelengths by the
Hubble Space Telescope.
Several groups of researchers from across the globe use the VLBA to study
stellar nurseries in our own Milky Way Galaxy and measure distances to
regions where new stars are forming. The key has been to improve measurement
accuracy to a factor of a hundred times better than that produced by the
highly successful Hipparcos satellite. Using small clouds of gas in
star-forming regions that strongly amplify radio waves, called cosmic
masers, the astronomers measured the tiny shift in the object's position in
the sky caused by the Earth's orbit around the sun. This, in turn, yielded
highly-accurate distances by the simple surveying technique of
triangulation, the "gold standard" of distance measuring techniques
available to astronomers.
"Knowing the distance accurately means we also know the luminosities, masses
and ages of the young stars much more accurately, and that is vital to
understanding how star formation works," Reid said. In addition, he pointed
out, the VLBA observations have shown the motions of the young stars in the
Milky Way are much more complicated than simple circular motion. Massive
young stars appear to be born orbiting the Milky Way considerably slower
than older stars. "This might be explained by the interaction of giant
molecular clouds, the ultimate sites of massive star formation, as they
"surf" spiral density waves in the Milky Way."
An international team of scientists led by Reid has used VLBI to detect the
slight change in apparent position of the object at the Milky Way's center
caused by our Solar System's orbit around that center. "It takes our Solar
System more than 200 million years to circle the center of our Galaxy, and
yet we can detect that motion in only a couple weeks with the VLBA -- truly
astounding!" Reid said.
The VLBA studies of the Galactic Center have shown that an object called
Sagittarius A* is at the exact gravitational center of our Galaxy. That
means, the scientists say, that the object must be incredibly massive. "The
VLBA measurements, combined with infrared observations of stellar orbits
around this object, provide overwhelming evidence that it's a supermassive
black hole," Reid explained. "These observations are also going to make it
possible to re-define the coordinate system used to map the entire Galaxy,"
Reid added.
Looking farther outward, astronomers achieved a longstanding goal of
measuring the spin of another galaxy. In 2005, Reid and his colleagues
measured both the rotational spin and the motion in space of the galaxy M33,
nearly 2.4 million light-years from Earth. Astronomers in the 1920s had
attempted such a feat, but their results were not accurate enough. "This
achievement had to wait for the VLBA," Reid said. This and subsequent work
has put strong limits on the amount of unseen "dark matter" around the giant
Andromeda galaxy, which M33 orbits. A continuing goal is to use VLBI
observations to measure the orbits of these and other galaxies within the
Local Group of galaxies to which our own Milky Way belongs.
In 1999, astronomers set a new standard for a distance measurement outside
the Local Group of galaxies when they used the VLBA to make a direct
geometric distance measurement to a galaxy called NGC 4258, 23.5 million
light-years from Earth. That measurement, accurate to within 7 percent,
caused other scientists to revise their indirect-measurement techniques for
the rest of the Universe. The NGC 4258 distance was calculated by measuring
the motion of masers in a disk of gas containing water molecules and
orbiting a supermassive black hole at the galaxy's center.
"Now, other galaxies are being observed in hopes of extending direct
distance measurement even farther out in the Universe," Reid said. "One
candidate, called UGC 3789, at a distance of about 160 million light-years,
will be measured with about 10 percent accuracy. Our goal is to further
improve these measurements and to measure 5 to 10 other galaxies in order to
determine the Hubble constant (the expansion rate of the Universe) to 3
percent accuracy. This would put limits on key parameters of the dark energy
that apparently is accelerating the expansion of the Universe," Reid added.
The kind of accurate measurement of distances and motions that VLBI
observations provide can benefit numerous other areas of astronomy, Reid
pointed out. For example, the distances to pulsars have been measured
directly with the VLBA, yielding better understanding of their
characteristics. The technique also could reveal planets circling some
nearby stars.
"Anytime you can do something as dramatic as improving measurement accuracy
by a hundredfold, you're bound to get a great scientific payoff," Reid said.
"We're looking forward to exciting new results in the coming years," he
added.
The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.
[NOTE: Images supporting this release are available at
http://www.nrao.edu/pr/2008/vlbiastrometry/ ]