The accompanying image suggests proximity of the open clusters M35 and NGC2158 but in reality the two lie at far distance from each other and their stellar populations could not be more different. The disparity in size is most obvious but is an illusion explained by the vast difference in distances of the two clusters. M35 (at upper left) is 2700 light years while NGC 2158 is much further from us around 15,000 light years.
On close inspection, the members of each
cluster reveal noticeable disparities in color.
The star colors underscore the distinctly different histories of the two clusters. The two clusters probably came into being with similar star populations, mostly blue and white massive O and B types with some lower mass yellow and orange types. M35 being comparatively young at 95 million years old, still maintains an abundance of blue and white massive stars. Even in this short time period however some red giants are visible which hint at the eventual fate of the rest of the cluster stars. In contrast NGC 2158 is over one billion years old, enough time to lose its population of O and B types to supernovas. All that remains in NGC 2158 are the lower mass, older population orange and yellow suns.
Most open clusters are loosely bound by
gravity and eventually disburse after a few hundred million years.
Because NGC 2158 is more compact than most clusters, gravity has
had a tighter grip on the individual stars and allowed it to become
an unusual ancient cluster.