From Clouds to Clusters: Are the Orion, Pleiades, and Hyades Stages of the Same Star Cluster?
The paper by Safaei et al. explores whether three well-known star clusters, the Orion Nebula Cluster (ONC), the Pleiades, and the Hyades, could represent different snapshots in the life of a single type of embedded star cluster. Using advanced computer simulations, the authors test whether a cluster that starts like the ONC could evolve to look like the Pleiades after about 100 million years, and like the Hyades after about 700–800 million years.
Introduction
Stars often form together in groups called star clusters, emerging from dense clouds of gas and dust. The Orion Nebula Cluster (ONC) is a very young, dense star-forming region about 500 light-years away, while the Pleiades and the Hyades are older, more spread-out clusters seen in the constellation Taurus. Because clusters begin very compact but appear much larger and looser as they age, this study asks a key question: could the ONC evolve into something like the Pleiades and then into something like the Hyades? To answer this, Safaei and colleagues build on earlier studies by running detailed simulations of star clusters as they form, lose gas, and interact with the Galaxy’s gravitational field.
Observational Data
The paper first summarizes what is known about these three clusters. The ONC is extremely young (about 2 million years old) and contains around 2,700 stars. It’s dense and compact, with a half-mass radius (the radius within which half the cluster’s mass lies) of about 0.8 parsecs. The Pleiades is about 110 million years old, contains roughly 1,000 stars, and is more spread out, with a half-mass radius closer to 3–4 parsecs. Finally, the Hyades is even older, about 800 million years old, with about 400 stars spread over a much larger region. These observed properties provide benchmarks for the simulations.
Simulations and Initial Conditions
The authors use a powerful simulation code called NBODY6, which calculates the motions of thousands of stars as they interact with each other and with the Milky Way. They set up models starting with star clusters like the ONC, compact, with about 4,000 stars, and include realistic physics such as binary stars, mass segregation (where heavier stars tend to sink to the center), gas expulsion, and the Galaxy’s tidal forces. The initial conditions were varied, especially the cluster’s mass and the degree of mass segregation, to see which combinations best reproduced the observations.
Results: The Path from ONC to Hyades
The simulations showed that clusters like the ONC lose a lot of their mass over time, primarily because of rapid gas expulsion and stellar evolution. Within 100 million years, they expand and lose enough stars to resemble the Pleiades, and by about 800 million years, they can look like the Hyades, much less massive, more dispersed, and closer to dissolving entirely. The authors find that models starting with about 4,200 stars (including brown dwarfs), high initial mass segregation, and compact sizes (half-mass radius around 0.25 parsecs) match the observed properties of the three clusters well.
Evolution of Cluster Properties
The simulations also track how the clusters’ key properties change. The tidal radius (where the cluster’s gravity balances the Galaxy’s pull) shrinks as the cluster loses mass. The half-mass radius and core radius both expand significantly as the cluster evolves. Initially compact and dense, the cluster becomes more extended and less centrally concentrated. The model labeled “M1.5k-S1”, with around 4,200 stars, strong mass segregation, and an initial mass of about 1,500 solar masses, fit the observations of all three clusters particularly well.
Conclusions
The results suggest that the ONC, Pleiades, and Hyades could indeed represent different stages in the evolution of a single type of embedded star cluster. Over hundreds of millions of years, the ONC could expand and lose stars to become like the Pleiades and eventually the Hyades. This study reinforces the idea that clusters form in very compact states and then expand as they lose gas and mass, filling their tidal radius over time. It also highlights the importance of gas expulsion and mass segregation in shaping cluster evolution.
Source: Safaei
