Why do star clusters provide excellent tests for theories of stellar evolution?
The star clusters are a great place to prove the theories related to stellar evolution. Over the period of Astronomical evolution we have studied a lot of stars and developed various theories related to how they are born, how they grow up to be main sequence stars, how they go supernova and then turn into a white dwarf or pulsars.
Stars are thought to be born out of a nebula. One nebula can result in thousands of stars which will spread all over with time. Open star clusters are proof of this theory. These contains young stars which were born out of same nebula and now slowly moving apart.
Globular star clusters tell us a lot about the older and massive stars of the galaxy.