The Spiral Arms and Interarm Separation of the Milky Way: an Updated Statistical Study

Measurements of the spiral arms (their pitch angle, number, shape, and the interarm separation) in the Milky Way have been published in the period from 1980 to early 2005, using different methods that yielded slightly or widely differing values, depending on hidden or unknown biases. A meta-analysis would be useful for searching for trends among these individual studies. First, we find a convergence for three parameters: the pitch angle, the number of arms, and the shape of the arms. Second, we find a change for the interarm separation near the Sun, a decrease of about 25%; it is our determination of these parameters that evolves, not the actual physical arms. Third, we recalibrate a recent arm model to take account of that gradual change and compare it with the spread in each arm parameter.