The trio of star clusters Orion Nebula, Pleiades and Hyades do not just randomly lie in the same region of space

Orion Nebula, Pleiades and Hyades: The latest research results indicate that these famous star clusters represent the different phases of life of one and the same star system. A team of astrophysicists at the Institute for Advanced Studies in Basic Sciences in Zanjan, Iran, and the University of Bonn have found evidence that these three star systems are not just located in roughly the same region of space but also developed in the same way.
These results were recently published in the journal “Monthly Notices of the Royal Astronomical Society.”

When the stars twinkle in the night sky, you can see the Seven Sisters
(Pleiades) and Hyades right next to one another with the naked eye, not far from the central “star” in Orion’s Sword. It isn’t really one star but rather the Orion Nebula Cluster – and thus the third of a group of star clusters.

However, the clusters in this trio are of different ages and at varying distances away from Earth. The Orion Nebula Cluster (ONC) is one of the youngest and most active star-forming regions in the Milky Way – at just 2.5 million years old and around 1,350 light years away. It contains thousands of young stars surrounded by the residual cloud of gas from which they were formed. In contrast, Pleiades, which is also known as the Seven Sisters, is about 100 million years old and its stars are much more widely scattered, while Hyades is around 700 million years old and contains fewer stars that are even more widely dispersed.

Baby, adolescent and elderly

“Our highly precise stellar dynamics calculations have now shown that all three star clusters originated from the same predecessor,” says Prof. Dr.
Pavel Kroupa from the Helmholtz Institute for Radiation and Nuclear Physics at the University of Bonn, a co-author of the publication. The Orion Nebula, Pleiades and Hyades are like three different photos of the same person.
These three photos show this entity at different ages: as a baby, adolescent and elderly.

It is a bit like if the same person was being born over and over again.
“From this we can learn that open star clusters seem to have a preferred mode of star formation,” says Kroupa, who is also a member of the transdisciplinary research areas “Modelling” and “Matter” at the University of Bonn. Does this mean that different molecular clouds form very similar star clusters? “It appears that there is a preferred physical environment in which stars form when they evolve within these clouds,” says the astrophysicist.

How does a young, dense star cluster like the Orion Nebula Cluster (ONC) develop into something like Pleiades over hundreds of millions of years and then ultimately mature into something resembling Hyades? The researchers headed by Dr. Ghasem Safaei from the IASBS (Institute for Advanced Studies in Basic Sciences) in Zanjan (Iran) are modeling this life cycle with the aid of powerful computer simulations.

The calculations concur with observed characteristics

Using these simulations, which include precise calculations of the varying forces between the stars, the researchers are able to follow the life cycle of a star cluster from its early formation in a gas-rich environment through to its gradual expansion and aging, when it loses gas and stars to its environment. The results concur very well with already observed characteristics such as the size, mass and structure of the Orion Nebula Cluster, Pleiades and Hyades at different phases of their lives.

“This research shows that it is entirely plausible that star clusters such as the Orion Nebula Cluster follow a development path that transforms them into systems like Pleiades and later on Hyades,” says Prof. Hosein Haghi from the IASBS, who is currently carrying out his research at the University of Bonn.

The team has simulated their development over a period of 800 million years, starting from an initial state based on the latest observed characteristics of young star clusters, including their compact size, high mass density and abundance of double stars. “The results show that clusters such as the Orion Nebula Cluster can lose up to 85 percent of their stars but nevertheless retain a coherent structure similar to Hyades, after they have passed through an intermediate stage similar to Pleiades,” explains Ghasem Safaei.

Interestingly, all three of these star clusters – Orion Nebula Cluster, Pleiades and Hyades – are located in the same region of the night sky. This fact has fascinated astronomers for a long time. The research team suspects that this is more than just a coincidence and could be related to how star clusters form and develop in the galaxy.

A delicate balance

Prof. Akram Hasani Zonoozi, one of the co-authors of the study, adds: “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.” Prof.
Hasani comes from the IASBS and is currently carrying out her research at the University of Bonn with funding from the Humboldt Foundation.

This research is not only shedding more light on the life cycle of star clusters. The researchers have also demonstrated how well modern theoretical simulations can be combined with real observations. It will open up new possibilities for understanding the origins of other star clusters and refining models that show how stars and their related environments develop over time.

Publication: Ghasem Safaei, Hosein Haghi, Akram Hasani Zonoozi and Pavel
Kroupa: Are the ONC, Pleiades, and Hyades snapshots of the same embedded cluster?, Journal “Monthly Notices of the Royal Astronomical Society”, DOI:
https://doi.org/10.1093/mnras/staf941, arXiv:
https://arxiv.org/abs/2507.12531