[petuniasevan]

The presence of bright, blue stars is a benchmark for star cluster youth. (Relatively speaking, of course). Stars that are hotter are bluer, and their fusion processes run much faster than those of smaller stars (some of the processes differ too, but I digress...). Thus they burn bright, and burn out early.

Other clusters show a greater age by the presence of red giants. The beautiful Jewel Box Cluster contains a lot of blue stars and ONE red giant, like a ruby among sapphires on black velvet. One star has reached the end of its ordinary "fuel" (hydrogen) in its core and now has to convert layers around the core, so it expands and the outer layers of the star expand and cool.

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[petuniasevan]

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