Current theory and observations suggest that clusters almost always form from a single, very large cloud of gas. All of the stars in the cluster form over the same short period of time (anywhere from a few thousand to a few million years). Open clusters are fairly young structures and usually dissipate from the random motions of the stars after a few hundred million years, or a few billion years at most. Globular clusters stick together tightly, by contrast, and can last for many billions of years.