An intuitive 3D map of the Galactic warp's precession traced by classical Cepheids

The Milky Way's neutral hydrogen (Hi) disk is warped and flared(1,2). However, a dearth of accurate H-i-based distances has thus far prevented the development of an accurate Galactic Disk model. Moreover, the extent to which our Galaxy's stellar and gas disk morphologies are mutually consistent is also unclear. Classical Cepheids, primary distance indicators with distance accuracies of 3-5% (ref. (3)), offer a unique opportunity to develop an intuitive and accurate three-dimensional picture. Here, we establish a robust Galactic Disk model based on 1,339 classical Cepheids. We provide strong evidence that the warp's line of nodes is not oriented in the Galactic CentreSun direction. Instead, it subtends a mean angle of 17.5 degrees +/- 1 degrees (formal) +/- 3 degrees (systematic) and exhibits a leading spiral pattern. Our Galaxy thus follows Briggs' rule for spiral galaxies4, which suggests that the origin of the warp is associated with torques forced by the massive inner disk(5). The stellar disk traced by Cepheids follows the gas disk in terms of their amplitudes; the stellar disk extends to at least 20 kpc (refs. (6,7)). This morphology provides a crucial, updated map for studies of the kinematics and archaeology of the Galactic Disk.