Globular star clusters are different in size and nature from open star clusters. Where open clusters contain a few dozen to a few hundred new stars, globular clusters are each populated by hundreds of thousands of the oldest stars in the universe.
Globular clusters formed 12-13 billion years ago, not long after the universe began. They likely collapsed from clouds of gas too small to form a galaxy but too large to form an open star cluster. In a sense, globular clusters are a little like “micro-galaxies” left over from the formation of larger galaxies.
Our Milky Way retains some 180 globular clusters. In the early 1900’s, famed astronomer Harlow Shapley figured out most globular clusters in the Milky Way are found in a halo around the nucleus of our galaxy. He used the distance and position of globular clusters to determine the size of the Milky Way and the Sun’s position on its outskirts.
Because they’re so old, stars in globular clusters are quite different from younger stars in open clusters. Younger stars contain traces of heavier elements like calcium, silicon, iron, and carbon… what astronomers call “metals”. But the stars in globulars formed long before such elements were formed in the innards of massive old stars. The stars in globulars are called “metal poor” and consist almost entirely of hydrogen and helium.
Unlike open star clusters, globular clusters are strongly bound by gravity and are stable over time. Eventually, most of the stars in globular clusters will die off and fade from view. But as far as we know, globular clusters… even as they darken… will remain bound forever.
You can see dozens of these spherical, tightly-bound clusters with binoculars or a small telescope. In the northern hemisphere, the brightest and prettiest globular clusters include the Great Cluster in Hercules, also known as M13, and the clusters M3 and M5 in the constellations Canes Venatici and Serpens, respectively. The two brightest globular clusters in the sky, the Omega Centauri cluster and 47 Tucanae (in Centaurus and Tucana, respectively) are only visible south of the tropics.
As you learn to observe globular clusters, you might at first think they’re all the same: just fuzzy, grainy white balls in the eyepiece of your telescope. But as you look closer, you’ll see that each differs in shape and structure, as distinct as a human face.