There are many published values for the pitch angle of individual spiral arms, and their wide distribution (from -3° to -28°) begs for various attempts for a single value. Each of four statistical methods used here yields a mean pitch angle in a small range, between -12° and -14° (Table 7, Fig. 2). The final result of our meta-analysis yields a mean global pitch angle in the Milky Way's spiral arms of -13.1° ± 0.6°.
Monthly Notices of the Royal Astronomical Society450, no. 4 (21 May 2015): 4277–4284.