Periodic Variables as Distance Indicators in the Gaia era

Periodic variable stars, such as Cepheids, RR Lyrae, eclipsing binaries, and long-period variables, are important distance indicators. Historically, distances to some open clusters, globular clusters, Milky Way dwarf galaxies, and nearby galaxies are well determined based on these distance indicators. However, both the rapid development of large, wide-eld instruments and new insights oered by infrared facilities have stimulated research of variable stars. The role of variables in the context of the distance scale should be updated. With the unprecedented Gaia second Data Release (DR2) parallaxes, the precision of the W UMa-type contact binary period–luminosity relation (PLR) is better than 6%. The signicant number of the contact binaries makes them an important distance tracer. The known number of classical Cepheids in the Milky Way’s disk has been increasing rapidly in recent years. Based on Cepheid distances, a new 3D map of the MilkyWay’s stellar disk has been constructed that is both intuitive and complete. In the outer disk, the stellar disk agrees with the gas disk in terms of their amplitudes. The outer disk is warped, and the morphology is complicated since the warp exhibits precession. In the next three years, new periodic variables will be detected. Based on a large sample of these variables, the distance and structure of our Milky Way, the Local Group, and nearby galaxies will be better understood.