Headlines > News > Galaxy's Neighboring Spiral Arm Is Closer Than Thought

Galaxy's Neighboring Spiral Arm Is Closer Than Thought

Published by Sigurd De Keyser on Thu Dec 29, 2005 10:18 pm
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Mark Reid and his colleagues measured the distance to the Perseus spiral arm and found it to be closer than believed, only 6400 light-years away. Credit Y. Xu et al.The Perseus spiral arm – the nearest spiral arm in the Milky Way outside the Sun’s orbit – lies only half as far from Earth as some previous studies had suggested. An international team of astronomers measured a highly accurate distance to the Perseus arm for the first time using a globe-spanning system of radio dishes known as the Very Long Baseline Array (VLBA), which offers the sharpest vision of any telescope in existence. Additional VLBA measurements will help astronomers to determine the true structure of the Milky Way.

“We know less about the structure of our own galaxy than we do about many nearby galaxies like Andromeda,” said CfA astronomer and team leader Mark Reid. “We literally can’t see the forest for the trees because we are embedded inside our own galaxy, and interstellar dust blocks our view.”

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, while observations comparing the apparent brightness of massive, young stars with estimates of their intrinsic brightness yielded a distance of only about 7,200 light-years. The new VLBA measurements confirm with an accuracy of 2 percent that the Perseus spiral arm is located about 6,400 light-years from the Earth.

“Our neighbors are closer than we thought,” stated first author Ye Xu (Shanghai Astronomical Observatory).

Obtaining accurate distances in astronomy is a difficult challenge. The most reliable method for measuring astronomical distances is called trigonometric parallax, a technique similar to the triangulation used by land surveyors. A trigonometric parallax is determined by observing the change in position of a star relative to a very distant, essentially fixed object like a quasar, as the Earth moves in its orbit around the Sun. The parallax method is powerful but requires exceptional accuracy.

“I have spent more than a decade developing the calibration techniques we needed to obtain this result,” said Reid.

The team achieved an accuracy of 10 micro-arcseconds, which is a factor of 100 better than previous methods. 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 VLBA is the only telescope able to provide such high resolution.

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