Astronomers have resolved a long-standing debate about how far our sun is from a nearby cluster of stars in the Milky Way. The new measurement may help scientists more accurately map out the shape of the galaxy, as well as determine the amount of gravitational "muscle" its star-filled arms contain.
The Milky Way is composed of several spiral arms--long, thin bands of bright, young stars that fan out from the center like the blades of a pinwheel. Our sun is located in the rather short Orion spiral arm, which is tucked inside the larger Perseus spiral arm. But astronomers aren't sure how far away the Perseus arm is, and knowing that could help them determine the true size and makeup of the Milky Way. Two separate measurements of Perseus' distance have given values that differ by a factor of 2--a large discrepancy even by astronomy standards, says Mark Reid of the Harvard-Smithsonian Center for Astrophysics.
Reid and his colleagues decided to resolve the issue by using the most direct way to measure distance short of pulling out a very, very long ruler. The method, called triangulation, is analogous to gauging your distance to a chair by shifting your head left and right, while noticing how the chair moves relative to an object even farther away. But instead of wagging their heads, the researcher used the motion of the Earth around the sun to give them different perspectives on a star-forming region in Perseus called W3OH. Over the course of a year, Reid and company used the well-scattered radio telescopes of the Very Large Baseline Array to track W3OH's position relative to three background quasars. The amount of change, or parallax, indicated the Perseus spiral arm is 6360 light years from the Sun, the team reports online 8 December in Science.
The new distance is superior to the previous estimates, as it does not rely on assumptions about the brightness of certain sources or the rotation speed of the galaxy, says radio astronomer Philip Diamond of the University of Manchester in the United Kingdom.
The researchers also found that, compared to most stars, W3OH is whizzing around the galaxy more slowly, perhaps because Perseus's gravity is tugging on it. That might indicate that the galaxy's arms are denser than their surroundings, as predicted by the-so-called spiral density-wave model. Such accurate measurements bring "hope that a precise map of the spiral structure of the Milky Way will be built soon," says astrophysicist Benoit Famaey of the Université Libre de Bruxelles, Belgium.
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