The spiral structure of the Milky Way

The morphology and kinematics of the spiral structure of the Milky Way are long-standing problems in astrophysics. In this review we firstly summarize various methods with different tracers used to solve this puzzle. The astrometry of Galactic sources is gradually alleviating this difficult situation caused mainly by large distance uncertainties, as we can currently obtain accurate parallaxes (a few μas) and proper motions (≈1 km s^-1) by using Very Long Baseline Interferometry (VLBI). On the other hand, the Gaia mission is providing the largest, uniform sample of parallaxes for O-type stars in the entire Milky Way. Based upon the VLBI maser and Gaia O-star parallax measurements, nearby spiral structures of the Perseus, Local, Sagittarius and Scutum Arms are determined in unprecedented detail. Meanwhile, we estimate fundamental Galactic parameters of the distance to the Galactic center, R ₀, to be 8.35 ± 0.18 kpc, and circular rotation speed at the Sun, Θ₀, to be 240±10 km s^-1. We found kinematic differences between O stars and interstellar masers: the O stars, on average, rotate faster, >8 km s^-1 than maser-traced high-mass star forming regions.