An open cluster is a loose group of a few dozen to a few thousand stars that formed together from one molecular cloud and remain loosely bound. They populate the disk of the galaxy, are typically young, and are far less tightly packed than globular clusters. Because their members share an age and composition, they are natural laboratories for testing how stars of different masses evolve. Gravity eventually disperses them, and the Sun almost certainly began life in one whose members are now scattered across the galaxy.