What Is An Open Star Cluster? (EXPLAINED!)

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An open star cluster is a group of stars bound together by mutual gravitational attraction. They are usually found in the spiral arms of galaxies or within irregular galaxies and contain anywhere from a few dozen to several thousand stars. 

Open star clusters are important because they provide insight into the formation and evolution of galaxies, as well as clues about our own Milky Way galaxy. Discover how and where they form and what happens as they reach the end of life.

How Are Open Star Clusters Formed?

An open cluster is one of the two main types of star clusters alongside globular clusters. The main difference between the two is that open star clusters are loosely bound and unstable, while globular clusters are far more tightly bound.

Open star clusters form when a giant molecular cloud collapses.

A giant molecular cloud is a cold, dense body of dust and gas that can hold thousands of times the mass of our Sun. Generally, they maintain equilibrium through rotation, magnetic fields, and turbulence, but several factors can interrupt this equilibrium and trigger a collapse:

  • Colliding with another cloud
  • Gravitational interactions with other bodies
  • A nearby supernova sending shockwaves

When this happens, the cloud can fragment into smaller clumps, such as infrared dark clouds – a particularly dense form of matter – which holds all the ingredients for star formation.

Only 30-40% of core gas forms stars, as residual gas expulsion, damages many infant stars, while others fade out of their own accord. This is a crucial stage for an open cluster as the remaining stars must possess strong enough gravity to keep them bound together – otherwise, they will become an unbound stellar association.

Once formation begins, the most massive (and hottest) stars, titled OB stars, emit extensive amounts of UV radiation, which ionizes surrounding gases and forms an H II region. Then, stellar wings (generated by these stars) begin to drive away the external gases until the system experiences its first supernova.

After a couple of tens of millions of years, the cluster will expel its entire gas supply, meaning no new stars can form.

All stars originate in multiple systems because of two factors. One, a cloud must contain many times the Sun’s mass to possess enough weight to collapse in on itself. But heavy clouds are unable to collapse into single stars.

Where Are Open Star Clusters Most Commonly Found?

We most commonly find open star clusters in irregular and spiral galaxies because these are the areas where new stars are being formed. Finding an open cluster in an elliptical galaxy would be unusual because they no longer create stars, and any lingering open clusters would have been destroyed.

We most commonly classify open clusters by a system invented by Robert Trumpler in the early part of the 20th century. Here, a Roman numeral from I-IV indicates the concentration of a cluster and its detachment from the star field surrounding it. Numerals 1-3 describe the brightness of the stars, while p, m, and r indicate a “poor,” “medium,” or “rich” amount of stars.

For example, the Pleiades open cluster is defined as I3rn – strong concentration of stars with a rich population (the n at the end tells us that the cluster is within nebulosity). In contrast, the Hyades are II3m, indicating fewer members, which are more dispersed.

How Many Open Star Clusters Are In The Milky Way?

Star Cluster

Scientists have discovered around 1100 open star clusters in the Milky Way so far, but they believe many more undetected ones could exist. Predictions suggest that the Milky Way will form one new open cluster every few thousand years.

One of the most brilliant clusters is Trumpler 14, whose striking array of glowing stars was captured in an image by the Hubble telescope. This cluster is located roughly 8,000 light-years from Earth in a region of the Milky Way that hosts one of the largest concentrations of massive stars.

Trumpler 14 is 500,000 years old (a baby compared to other open clusters, such as Pleiades, about 115,000,000 years old). Still, this young cluster forms stars at a remarkable rate giving onlookers a stunning display.

And these star clusters have a significant impact on their environment. As they eject high-speed particles from their surfaces, a strong wind surges out to space. As the winds collide with nearby material, it causes shockwaves that heat the gas and trigger intense x-ray bursts. In addition, they interact with nearby gas and dust clouds to kickstart the formation of new stars.

What Eventually Happens To Stars In An Open Star Cluster?

Any open star cluster will disperse in just a few million years because of its unstable nature, where the system’s escape velocity is lower than that of the average constituent star.

Encounters with other clusters and gas or dust clouds easily disrupt these clusters. An open star cluster can also lose celestial bodies through internal encounters.

Despite their (relatively) short lifespan, open clusters are significant in scientific studies because they help us to understand stellar evolution. This is because each star in the cluster possesses a similar age and composition, making it easier for scientists to study the effects of more subtle influences.


Open star clusters are loosely bound star gatherings created from the collapse of a giant molecular cloud. The unstable mass of these entities means that they are relatively short-lived and will disperse within a few million years.

Still, they are helpful to scientists because they contain stars of similar mass and composition, making it easier to study the effects of other, more subtle, influences on a star’s development.


Open cluster | ESA/Hubble | ESA/Hubble (esahubble.org)

Dazzling diamonds | ESA/Hubble (esahubble.org)

Open cluster (mcgill.ca)

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