If you plot the distribution of clusters, you discover that almost all of them are on one side of the sky, concentrated in the constellation Sagittarius in the summer sky.

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All the stars in a cluster are the same distance away, so you can compare brightness without having to worry about whether a star that appears faint is actually very bright but just far away.

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Globular clusters are old and give us a limit on the age of the universe. The universe couldn't be younger than the oldest star cluster.

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