Astronomy Practice Exam 2025 – Complete Preparation Guide

An open cluster is characterized by a loose, disorganized structure that usually contains a relatively small number of stars compared to other types of clusters, such as globular clusters. These stars are typically young, often formed from the same molecular cloud, which contributes to their relative proximity in age and composition. The lack of gravitational binding that is typical of open clusters allows them to spread out over time, leading to the loose and dispersed appearance that distinguishes them.

In contrast, tightly packed groupings of stars are indicative of globular clusters, which contain older stars and a more spherical shape. The incorrect options suggest characteristics that do not pertain to the nature of open clusters. Formations that have no visible stars or are restricted to binary systems do not align with the broader, more varied structure and age range typically seen in open clusters. Thus, the distinct traits of open clusters, including their loose organization and the predominance of younger stars, effectively define them within the classification of star clusters.