What distinguishes a globular cluster from an open cluster?

Globular clusters are indeed distinguished by their age relative to open clusters. They are typically much older, containing some of the universe's oldest stars. These clusters consist of hundreds of thousands to millions of stars that are tightly bound by gravity, which contributes to their overall antiquity. As such, the stars within globular clusters have typically gone through many stages of stellar evolution, resulting in a composition that is relatively rich in old, red stars known as giants and supergiants.

In contrast, open clusters are generally younger, containing fewer stars (usually ranging from a few dozen to a few hundred) that are more loosely bound and can disperse over time. They often contain a mix of young, hot stars and various amounts of gas and dust, which allows for ongoing star formation. This age difference is one of the key factors that scientists use to classify these two types of star clusters.