How is the color of a star cluster linked to its age?

Measuring a star cluster’s color yields information about its age. A newly formed cluster of stars contains stars of all colors, but it appears blue because its hot, blue stars outshine all the others. These blue stars are relatively short-lived compared to the cluster’s dimmer, redder stars. The blue stars die off over a period of a few hundred million years, but the redder stars continue to shine for billions of years. Therefore, as a star cluster ages, its color gradually shifts from blue to red. The oldest, reddest star clusters we have found are over 10 billion years old.

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