If you want to see how people grow, you can either watch a person for a lifetime or you can look at different populations of peoplefrom babies to old peopleand compare them.

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.

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.

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.

If you want to see how people grow, you can either watch a person for a lifetime or you can look at different populations of people—from babies to old people—and compare them.

If you want to see how people grow, you can either watch a person for a lifetime or you can look at different populations of people—from babies to old people—and compare them.