Why do open clusters have a loose appearance?
Most of the brightest stars are older yellow stars, but a few young blue stars are also visible. This picture is a composite of three images taken by the Hubble Space Telescope.
Open clusters are described as having a loose appearance primarily due to their small total mass and their relatively large core radii.1
Unlike globular clusters, which are massive and tightly bound, open clusters possess a lower concentration of stars, typically ranging from only tens to a few thousand members. This structural looseness, combined with their low mass, means they are not sufficiently gravitationally bound to resist the disruptive tidal forces within the Milky Way Galaxy. Consequently, these clusters are generally short-lived, as these tidal effects eventually cause them to disperse.12