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There are thousands of stellar clusters in our Galaxy. We can
quite easily determine the age of all these clusters by putting the
stars onto an H-R Diagram and seeing what is the most massive star
still seen on the Main Sequence.
In the diagram above, which is the youngest cluster? Which is the
oldest? How can you tell?
If I told you that the Sun had a color index (B-V) of about 0.5, can you tell me how old is NGC 752?