Star Clusters

Explore Beautiful Batches of Stars

Black Holes Pictures Gallery - A Dying Star in Globular Cluster M15
Globular cluster Messier 15 shown in this color image Hubble's Wide Field Planetary Camera 2 (WFPC2). 40,000 light-years from Earth in the direction of Pegasus, M15 is one of nearly 150 known globular clusters that form a vast halo. NASA and The Hubble Heritage Team (STScI/AURA)

Star clusters are just what the name says they are: groupings of stars that can include anywhere from a few dozen to hundreds of thousands or even millions of stars! There are two general types of clusters: open and globular. 

Open Clusters

The open clusters, such as the Beehive in the constellation of Cancer and the Pleiades that grace the sky in Taurus, are groups born in the same area of space but are only loosely gravitationally bound together. Eventually, as they travel through the galaxy, these stars wander apart from each other.

Open clusters usually have up to a thousand or so members, and their stars are not more than 10 billion years old. These clusters are much more likely to be found in the disks of spiral and in irregular galaxies, which contain more star-forming material than older, more evolved elliptical galaxies. The Sun was born in an open cluster that formed about 4.5 billion years ago. As it moved through our rotating galaxy, it left its siblings behind long ago.

Globular Clusters

Globular clusters are the "mega-clusters" of the cosmos. They orbit the central core of our galaxy, and their thousands and thousands of stars are held together by a strong mutual gravity that creates a sphere or "globe" of stars. Generally speaking, stars in globulars are among the oldest in the universe, and they formed early in a galaxy’s history. For example, there are stars in globulars orbiting our galaxy's core that were born when the universe (and our galaxy) was quite young. 

Why Are Clusters Important to Study?

Most stars are born in these big batches within large stellar nurseries.Observing and measuring stars in clusters gives astronomers great insight into the environments in which they formed. Stars born recently often are more metal-rich than those that formed much earlier in history. Metal-rich means that they contain more elements heavier than hydrogen and helium, such as carbon and oxygen. If their birth clouds were rich in certain kinds of elements, then those stars will contain higher amounts of those materials. If the cloud was metal-poor (that is, if had a lot of hydrogen and helium, but very few other elements), then the stars it formed will be metal-poor. Stars in some globular clusters in the Milky Way are quite metal-poor, which indicates they formed when the universe was very young and there hadn't been time to form enough of the heavier elements. 

When you look at a star cluster, you're seeing the the basic building blocks of galaxies. Open clusters provide the stellar population of a galaxy's disk while the globulars hark back to a time when their galaxies were forming through collisions and interactions. Both stellar populations are clues to the ongoing evolution of their galaxies and of the universe.

For stargazers, clusters can be fantastic observation targets. A few well-known open clusters are naked-eye objects. The Hyades is another choice target, also in Taurus. Other targets include the Double Cluster (an pair of open clusters in Perseus), the Southern Pleiades (near Crux in the Southern Hemisphere), the globular cluster 47 Tucanae (a fabulous sight in Southern Hemisphere constellation Tucana), and the globular cluster M13 in Hercules (easy to spot with binoculars or a small telescope). 

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Petersen, Carolyn Collins. "Star Clusters." ThoughtCo, Aug. 27, 2020, Petersen, Carolyn Collins. (2020, August 27). Star Clusters. Retrieved from Petersen, Carolyn Collins. "Star Clusters." ThoughtCo. (accessed January 25, 2021).