How do massive star clusters survive for billions of years?
In short: Picture millions of stars born simultaneously in the same location. Held together by gravity, these celestial groups vary from youthful, evaporating collections to ancient, spherical giants. These massive clusters act as the universe's stellar dinosaurs, preserving cosmic history within their dense, tightly packed, and incredibly old stellar populations.
Star clusters are massive collections of stars that share a common origin, having formed at roughly the same time and location. The process begins when gravity causes clouds of gas and dust—primarily hydrogen—to collapse. As the pressure builds, the core heats up into a protostar, a stage that takes about a million years. Once a star is born, its lifespan varies wildly; while some massive O-type stars explode as supernovae within millions of years, others may survive for 10 billion years or more.
Astronomers distinguish between two primary types of clusters. Open clusters are relatively young groups containing hundreds or thousands of stars. Because they are less massive, they tend to evaporate over time, with stars eventually being ejected or slowly dispersing into space. In contrast, globular clusters are much larger and more spherical, containing anywhere from tens of thousands to millions of stars. These dense clumps can span between 50 and 450 light-years across and are often found in the Milky Way's outer halo.
Because globular clusters are so ancient, they serve as vital cosmic archives. Many formed between 8 and 13 billion years ago, making them a significant fraction of the age of the universe. These stars are typically redder and contain fewer heavy elements than younger stars. While they were once surrounded by gas, stellar winds, ultraviolet light, and supernovae eventually blew that material away, halting new star formation. Interestingly, while it is uncertain if they host planets, there is some evidence that globular clusters may have produced multiple generations of stars early in their lives.
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Sources
- Summary by Intellect · source: Star Clusters: Crash Course Astronomy #35
- Also: Star Clusters: Inside the Universe's Stellar Collections - NASA Science
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