Why do many open clusters eventually break apart?
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 groups of stars held together by mutual gravitation, but they are characterized by relatively small total masses and a loose structure. Because of these features, they lack the strong gravitational binding necessary to withstand the disruptive tidal forces present within the Galaxy.12
Over long periods, these tidal forces, combined with internal stellar encounters, cause the clusters to disintegrate as individual stars gradually "evaporate" from the group. Consequently, most open clusters are short-lived; only about half can survive these forces for more than 200 million years, and very few persist for as long as 1 billion years. As they age, their members eventually disperse, contributing to the general population of stars in the Milky Way Galaxy.324