3D spatial structure of Collinder 135, with ellipsoidal curves and its three projections. The green ellipsoid with its fitted center position (red pentagram) denotes the core region of the sample cluster, with the purple ellipsoid with its fitted center (blue pentagram) marking its outer structure. The small red and blue dots represent the members of the cluster in the core and outer regions, respectively. The different colored bars represent the different axes of the ellipsoids, respectively, as shown in the legend. The green and purple shaded areas in the three projections (XY plane, XZ plane, and YZ plane) indicate the core projections of the cluster and its shell projections, respectively.