M13

Just because an astronomical object is big and bright doesn’t mean it can’t maintain some mystery. The objects known as globular clusters, for example, are the biggest agglomerations of stars in the galaxy — hundreds of thousands of stars packed into a tight ball. That makes them quite easy to see and study. Even so, astronomers are still unsure how these balls of stars were born.

The Milky Way is home to fewer than 200 globulars. One of the best known is M13, the Hercules Cluster. It’s well up in the east at nightfall, in the constellation Hercules. Small telescopes reveal hundreds of its individual stars.

Globulars are the oldest objects in the galaxy. Their stars were born when the universe was quite young. Their most massive stars burned out a long time ago, leaving a population of stars that are no more massive than the Sun.

Despite decades of work, though, astronomers still haven’t agreed on a mechanism for making these clusters. In most globulars, the stars seem to have forme