The size and shape of our home galaxy, the Milky Way, is essential information because it contains the story of the structure and evolution of our cosmic neighborhood. But it also is important because our knowing the galaxy's size and shape enables astronomers to infer the distances to objects located within the galaxy. Distance, in turn, is often the main uncertainty when calculating a star or other object's mass from its apparent brightness. Conversely, knowing the precise distances to objects in the galaxy enables astronomers to assemble a coherent picture of the galaxy's size and shape.
The distances to nearby stars are precisely and accurately measured using the technique of parallax. When a celestial body is viewed from different, widely separated points, its position (in angle) with respect to far more distant background stars appears to be different: its parallax. Parallax is used to triangulate the distances to stars by measuring their apparent angular shifts six months apart, when the earth is on opposite sides of its orbit around the sun. But this traditional technique only works for relatively nearby stars for which the angular shifts are comparatively large. Still, the matter is so important that astronomers have tried for nearly a century (since the Milky Way was recognized to be a galaxy) to piece together a consistent picture.
SAO astronomer Mark Reid, together with a group of 13 colleagues, has perfected a new radio astronomy parallax technique that can measure bright objects as far away as 20,000 light-years or more. Writing in the latest issue of the Astrophysical Journal, the team presents a series of six papers (including one in press) on their new determinations of the distances to eighteen regions of massive star formation with extremely bright maser activity (masers are the radio analogs of lasers). The intense radiation allows the scientists to measure the parallax of these distant sources, and the team observed their apparent angular shifts over a period of one and one-half years.
The astronomers report new distances for objects that range from nearby, 1340 light-years, to very distant, 19,530 light-years. Their findings in some cases correct dramatic errors in previous distance estimates, as much as a factor of two. In addition, they compute a new distance to the center of the galaxy - 27,400 light-years - and they conclude that the galaxy probably has four spiral arms. The results are a landmark in distance measurements, and the first step in assembling a much more accurate picture of the size and shape of our cosmic neighborhood.