Star Cluster Coloring Page: Free Printable PDF Sheet

Star Clusters: Families of Stars Born Together

Stars That Share a Birthday

A star cluster forms when a single giant cloud of gas and dust collapses under its own gravity, breaking into many separate clumps that each ignite into a star at roughly the same time. Because every star in a cluster formed from the same original material within a relatively short span, astronomers use clusters as natural laboratories for studying how stars of different sizes age and change over time.

The Pleiades: A Cluster Visible to the Naked Eye

The Pleiades, also called the Seven Sisters, is one of the closest and most famous star clusters, sitting about 444 light-years from Earth and easily spotted as a small, tight group of stars in the winter sky. Cultures across the world, from ancient Greece to Japan to many Indigenous nations, developed independent legends and calendars around the Pleiades long before telescopes existed.

Open Clusters Versus Globular Clusters

Astronomers divide star clusters into two main types: open clusters, which are loosely bound groups of a few hundred to a few thousand relatively young stars, and globular clusters, dense spherical swarms that can pack hundreds of thousands of much older stars into a surprisingly compact region. The Milky Way galaxy hosts roughly 150 known globular clusters, some containing stars nearly as old as the universe itself.

Why Clusters Eventually Drift Apart

Open clusters are only loosely held together, and over hundreds of millions of years the gravity of the surrounding galaxy gradually pulls individual stars away from the group, a slow process called cluster dispersal. Our own sun is thought to have formed inside a star cluster billions of years ago, meaning it likely once had many sibling stars that have long since scattered across the galaxy.

Counting Stars Astronomers Once Thought Were Few

Early stargazers with only their eyes could see just a handful of individual stars inside a cluster like the Pleiades, but Galileo's telescope observations in 1610 revealed dozens more stars packed into the same small patch of sky than anyone had previously counted. Modern telescopes have since resolved thousands of individual stars in the same cluster, showing just how much extra detail better equipment reveals.

Clusters as Cosmic Age Clocks

Because every star in a cluster formed at nearly the same time, astronomers can estimate a cluster's total age by studying how its brightest, most massive stars have already started to change, since bigger stars burn through their fuel and evolve faster than smaller ones. This technique has helped scientists calculate ages for some globular clusters exceeding 12 billion years, among the oldest known objects in the universe.

Hubble's Deep Look Into Crowded Clusters

The Hubble Space Telescope has photographed globular clusters in extraordinary detail, resolving individual stars packed so closely together that ground-based telescopes on Earth simply blur them into a single glowing blob. Those crisp images have let astronomers study crowded star systems that would otherwise be impossible to examine star by star from Earth's surface.