Star Clusters Explained: Open vs Globular

Star clusters are among the most rewarding objects for amateur astronomers, bright, beautiful, and visible in everything from binoculars to large telescopes. But not all clusters are created equal. The two main types, open clusters and globular clusters, differ in nearly every way: age, size, location, number of stars, and appearance. Understanding the difference deepens every observation.

Open Clusters: Stellar Nurseries

Open clusters (also called galactic clusters) are loose collections of a few dozen to a few thousand stars that formed together from the same molecular cloud. They're young by astronomical standards, most are less than a billion years old, and many are only a few million years old. They contain hot, bright, blue stars alongside fainter members.

Because they're young and formed from the same gas cloud, open clusters give astronomers a unique laboratory: all the stars are the same age, same distance, and same chemical composition. The only thing that differs is their mass. This makes them ideal for testing theories of stellar evolution.

Famous examples you can observe include:

• The Pleiades (M45): About 444 light-years away, roughly 100 million years old. Best in binoculars. Read more in our Taurus guide.
• The Hyades: The nearest open cluster at 150 light-years. About 625 million years old.
• The Beehive (M44): A rich cluster in Cancer, easily visible to the naked eye under dark skies.
• The Double Cluster (NGC 869/884): Two clusters side by side in Perseus. Stunning in binoculars or a wide-field telescope.

Globular Clusters: Ancient Spheres

Globular clusters are a completely different beast. They're massive, dense, spherical collections of hundreds of thousands to millions of stars, bound together by gravity into tight balls. They're ancient, most are 10 to 13 billion years old, nearly as old as the universe itself. They orbit the center of our galaxy in a halo, rather than sitting in the galactic disk like open clusters do.

The stars in globular clusters are predominantly old, red, and metal-poor. They formed in an era before the galaxy had been enriched with heavy elements from generations of supernovae. Looking at a globular cluster, you're seeing some of the oldest surviving stars in the Milky Way.

Top globular clusters for amateur astronomers:

• M13 (Great Cluster in Hercules): About 25,000 light-years away, containing roughly 300,000 stars. Resolves beautifully in an 8-inch telescope.
• M22: One of the nearest globulars, in Sagittarius. Bright and partially resolved even in small telescopes.
• Omega Centauri (NGC 5139): The largest globular cluster visible from Earth, arguably a stripped dwarf galaxy core. Spectacular from southern locations.
• M5: A slightly underrated gem in Serpens. Rivals M13 in beauty and resolves well in modest telescopes.

Side-by-Side Comparison

How to Observe Both Types

Open clusters and globular clusters reward different observing approaches:

• Open clusters often look best at low magnification. Many span half a degree or more, so a wide-field eyepiece or binoculars give the best sense of the cluster as a whole. High magnification zooms in too far and you lose the forest for the trees.
• Globular clusters benefit from higher magnification. Start at low power to find the cluster, then increase to 150-200x to resolve individual stars around the periphery. The contrast between the dense, unresolved core and the sparkling resolved edges is what makes globulars visually compelling.

For a curated list of the best targets to start with, check our guide to the best Messier objects for beginners.