A Cosmic Family Tree: The Orion Nebula, Pleiades, and Hyades
Recent scientific discoveries have unveiled a fascinating connection between three famous star clusters — the Orion Nebula Cluster, the Pleiades, and the Hyades. These clusters, though distinct in their current appearances, are now believed to be different stages in the life of a single evolving system. This revelation comes from advanced simulations that show how these clusters transition through various phases of development.
The Orion Nebula Cluster, located approximately 1,350 light-years away, is relatively young at just 2.5 million years old. It contains around 4,000 stars packed into a dense, gas-rich region. Over time, these clusters lose mass due to powerful stellar winds, ultraviolet radiation, and supernova explosions, which push away the leftover gas and cause the cluster to expand rapidly.
From Cradle to Dispersal
As stars form within these compact clouds, energy from massive stars begins to expel the gas that helped create them. This early gas expulsion, occurring within the first 10 million years, leads to a dramatic shift. Clusters like the Orion Nebula become “supervirial,” meaning they hold too much kinetic energy to stay tightly bound. As a result, they start to expand.
Computer models suggest that by 100 million years — the current age of the Pleiades — the cluster loses about 53% of its stars but retains a stable core. After roughly 700 million years, what remains resembles the Hyades cluster: a loose group of older stars with only 9% of the original members left.
These long-term simulations, powered by advanced computational models, help researchers track how star clusters change under the influence of internal dynamics and external forces, such as the pull from the Milky Way’s tidal field.
Snapshots Across Cosmic Time
The Orion Nebula, Pleiades, and Hyades can be thought of as three snapshots in a stellar lifetime. Professor Pavel Kroupa of the University of Bonn explained it this way: “The Orion Nebula, Pleiades, and Hyades are like three different photos of the same person — as a baby, adolescent, and elderly.”
All three clusters lie close together in the sky. The Orion Nebula sits near Orion’s Sword. The Pleiades, also known as the Seven Sisters, appear as a small, bright grouping of stars. The Hyades form a V-shape in the constellation Taurus. Their proximity has long intrigued astronomers.
Despite their differences in size, age, and structure, these clusters likely originated from the same formation process. The findings suggest that star clusters don’t form randomly but follow a preferred path, shaped by the physical conditions of their birth clouds.
Testing Star Formation Models
To understand this better, scientists looked at two leading theories for how star clusters form. The first is hierarchical assembly, where small groups of stars merge over time to form larger clusters. The second is monolithic collapse, where a dense region in a molecular cloud rapidly forms a large cluster in one event.
Simulations by Banerjee and Kroupa in 2015 showed that the hierarchical model doesn’t work for clusters like NGC 3603 or R136. These clusters form too quickly and with too much structure for smaller groups to have time to merge. Instead, the monolithic model fits better, suggesting clusters form in one burst and then rapidly evolve as gas is lost.
Further studies by researchers like Zonoozi and Marks found that mass segregation — the way heavier stars settle toward the center — also shapes the fate of a cluster. Clusters with more massive stars at birth tend to dissolve faster and grow larger over time.
Looking Deeper into the Sky
This research also highlights how well computer models can match real-world observations. Combining detailed simulations with telescope data, scientists can now reconstruct the life story of star clusters from their formation to their slow dispersal.
Prof. Akram Hasani Zonoozi, a co-author of the study, explained: “This research gives us a deeper understanding of how star clusters form and develop and illustrates the delicate balance between internal dynamics and external forces such as the gravitational pull of the Milky Way.”
As star clusters like the ONC expand and lose stars, their shapes and brightness change. Over hundreds of millions of years, the transformation is striking. Yet their shared origin gives a clearer picture of how the galaxy shapes its stars.
This work doesn’t just solve a mystery about three well-known clusters. It also helps refine models of how stars and galaxies evolve. These findings may even lead to better predictions about the life cycles of other star groups we observe throughout the universe.