The most massive stars burn out the fastest, and become red giants sooner. We can't watch this happen, since it takes millions of years. But we can look for examples of clusters of various ages, and compare their HR diagrams. Two types of star clusters Star clusters are ideal for studying stellar evolution. Few hundred to a few thousand stars Loosely organized, irregular shape.
Brightest stars are usually blue main sequence stars.
May be some red giants. Ages from HR diagrams range from less than a million years up to several billion. Very few open clusters older than the age of our sun. Tens of thousands to millions of stars. Symmetrical, spherical shape, concentrated to center in most cases.