Which type of cluster is typically classified as "young"?

Open clusters are categorized as "young" because they consist of stars that have recently formed, generally within the last few million years. These clusters are typically found in the arms of spiral galaxies and contain a few dozen to a few thousand stars, which share a common origin and are gravitating together due to mutual attraction.

The stars in open clusters are usually hotter, brighter, and more massive than those found in older clusters. As a result, they often feature a mix of early-type (blue) and late-type (red) stars, with a predominance of young, luminous stars. Over time, gravitational interactions with the surrounding material and other clusters, as well as the dynamic processes within the galaxy, lead to the dispersal of the stars in the open cluster. This characteristic of being relatively young and containing many bright stars sets open clusters apart from globular clusters, which are older and contain much older stars in a more spherical arrangement.