The theory of stellar evolution was developed by arranging (on paper) different types of stars according to their color and absolute brightness—what is called a Hertzsprung-Russell diagram. A physical rationale was then devised for how stars moved from one position on the diagram to another. Supposedly, a star’s age was determined by its place on the diagram. However, astronomers recognize that all stars in each massive star cluster formed at about the same time, because the stellar wind from the first stars to form would have blown out of the tight cluster the raw material needed to form all the other stars in the cluster. Despite the same age for stars in a given cluster, the Hertzsprung-Russell diagram sometimes gives drastically different ages.a