Star Formation in the H II Region Sh 2-205: 3D Morphology and Kinematics from Young Stars and Molecular Gas

Using Gaia astrometry of young stars combined with CO observations, we present the first systematic three-dimensional (3D) analysis of the structure, kinematics, and evolutionary history of the star-forming regions in the environs of the H ii region Sh 2-205 (S205). S205 exhibits a complex morphology and coherent expansion on both global and subregional scales. We identify several O9-B1 stars and a 0.56 Myr old pulsar that are likely associated with the region. A momentum estimate suggests that feedback from these objects may account for the observed overall expansion. Trace-back analysis of the expansion, combined with color–magnitude diagram fitting for young star clusters, indicates at least two episodes of star formation. These results reveal a complex star formation history of S205 and provide new insights into its 3D evolution.