Locations of Gaia DR2 OB (O–B2) stars (red circles) and masers (triangles) (Reid et al. 2014; Xu et al. 2016). The formal parallax uncertainties of the OB stars shown here are better than 10%, but do not include a possible ±0.1 mas systematic error. The Galactic center (red star) is at (0,0) and the Sun (Sun symbol) is at (0, 8.31). The solid and dashed curved lines denote the center and ±1σ widths of spiral arm models based on VBLI maser parallaxes. The Perseus arm (black), the Local arm (blue), and Sagittarius arm (green) are shown. Straight dashed lines indicate Galactic longitude.