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Galaxy’s spiral arm closer than was thought, research finds
Dec. 28,
2005
Courtesy Harvard-Smithsonian Center for Astrophysics
and World Science staff
The nearest spiral arm of our galaxy—after the one
we inhabit—is half as far off as some studies previously suggested, new research has found.
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A schematic map of our Milky Way galaxy
based on the limited amount of information known. (Courtesy the The Canadian Galactic Plane Survey)
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A team of astronomers measured the distance to the so-called Perseus arm using a worldwide system of radio telescopes, known as the Very Long Baseline Array. Additional measurements with the same system will help astronomers map our Milky Way galaxy, they added.
“We know less about the structure of our own galaxy than we do about many nearby galaxies like Andromeda,” said team leader Mark Reid of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
“We literally can’t see the forest for the trees because we are embedded inside our own galaxy, and interstellar dust blocks our view.”
The findings were published in the December 8, 2005 advance online issue of the research journal
Science.
Previous estimates of the distance to the Perseus arm varied by a factor of two. Studies based on the motions of stars yielded a distance of more than 14,000 light-years; a light-year is the distance light travels in a year. Other research based on the dimming of starlight with distance yielded a distance of about 7,200 light-years.
The new measurements show the spiral arm is about 6,400 light years from the Earth, with a 2 percent accuracy, the astronomers said. “Our neighbors are closer than we thought,” said team member Ye Xu of Shanghai Astronomical Observatory.
Our solar system lies in an arm called the Orion Arm or Local Arm.
Obtaining accurate distances in astronomy is a difficult challenge. The most reliable method is called parallax. This involves measuring the distance a star seems
to move across the sky as our Earth travels around the sun. The further the star seems to move, the closer it is, just as trees by a roadside whip by a traveling car faster than the far-off mountains do because they’re closer.
But parallax only works if the measuring instruments are very accurate, and that was what the astronomers achieved in this case, said Reid: “I have spent more than a decade developing the calibration techniques we needed” for this study.
The measurement was accurate to within one 360-millionth of a degree across the sky, a factor of 100 better than previous methods, Reid contended. That resolution is equivalent to looking from the Earth to a person standing on the Moon’s surface and telling whether that person is holding a flashlight in their right or left hand.
The Very Long Baseline Array is the only telescope system that can provide such high resolution, added Reid, whose team used the system to examine the region near a newly formed star in the Perseus arm called W3OH.
Radio telescopes gather light from the radio part of the light spectrum, a type of light that is less energetic than visible light.
Reid’s team gathered radio waves from bright sources of radio waves, known as methanol masers. Masers are regions in clouds of molecules that can amplify, or strengthen, radio-wave emission the same way lasers amplify light emission.
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Front image: artist's impression of the
Milky Way galaxy as seen from a distance (Credit: NASA/JPL-Caltech/R. Hurt [SSC/Caltech])
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