Final answer:
Stars in star clusters are assumed to have formed at the same time, from the same cloud, and have similar composition, differing mainly in their mass. This mass difference causes them to age at different rates, making clusters ideal for studying stellar evolution. There are three types of star clusters: globular, open, and associations, each with distinct characteristics.
Step-by-step explanation:
Stars in a star cluster tend to have certain stellar properties in common, particularly that they are believed to have formed from the same cloud of interstellar matter, nearly at the same time, and thus have similar composition. This common origin means that the stars within a star cluster will predominantly differ only in one main aspect: their mass. It's the mass of the stars that significantly influences how quickly each star progresses through the various stages of its life, such as the main sequence, giant phase, and so on.
There are three basic types of star clusters known to astronomers: globular clusters, open clusters, and stellar associations. Globular clusters are collections of very old stars, forming a spherical shape, and are found in a halo around galaxies. In contrast, open clusters and associations contain younger stars and are often found in the plane of the Galaxy or in regions of gas and dust, respectively.
Because all stars in a cluster are of about the same age but vary in mass, they provide an excellent natural laboratory for studying evolutionary effects in stars. Different masses lead to different stages in their lifecycles, even though they are of similar age, thereby helping astronomers understand stellar evolution.