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. |
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. |
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. |