Stars usually form in clusters, and such clusters can also form in pairs or groups. Binary clusters (BCs), defined as pairs of open clusters closely associated both in position and kinematics. Their formation records how stars arise within giant molecular clouds, making them important tracers of star formation and cluster evolution.

Recently, PhD candidate LIU Guimei and her supervisor Prof. ZHANG Yu from the Xinjiang Astronomical Observatory (XAO), Chinese Academy of Sciences, together with their collaborators at the Shanghai Astronomical Observatory (SHAO), carried out a large, systematic study of binary open clusters in the Milky Way.

Using high-precision astrometry from the Gaia satellite and applying uniform, stringent selection criteria, the researchers constructed a large sample of candidate BCs, confirming 400 candidate systems, 268 of which are newly reported. The results have been published in Astronomy & Astrophysics.

In this work, the researchers analyzed nearly 4,000 high-quality open clusters using Gaia DR3 astrometry and kinematics. They established a statistical, quantitative criterion for spatial and velocity proximity and validated it against randomized mock samples. With this framework they identified 400 BC candidates and classified them into three categories (Fig. 1): (i) primordial binary clusters (co-natal), (ii) tidal-capture/resonant-capture binary clusters, and (iii) optical pairs (chance alignments).

Further analysis shows that 61% of the candidate binary clusters are highly consistent in age and kinematics, supporting formation from the same giant molecular cloud; 83% display significant tidal interactions. The interaction strength correlates clearly with spatial separation—the closer the pair, the stronger the mutual attraction and perturbation.

Overall, about 17% of open clusters are currently in binary or multiple-cluster systems, and roughly 10% likely formed as primordial binary clusters; these fractions agree well with previous theoretical and observational estimates. 

Fig 1. Representative types of binary clusters. Panels show the spatial distributions (left), radial velocity (middle), and the color–magnitude diagram (right). Arrows indicate the clusters’ tangential velocities.

Cross-matching with previously reported BCs shows that the method recovers a large fraction of known systems. Despite stricter selection criteria, it also adds 268 newly identified physical BCs to the Galactic sample (Fig. 2).

Fig 1. Representative types of binary clusters. Panels show the distributions in spatial coordinates (left), radial velocity (middle), and the color–magnitude diagram (CMD) (right). Arrows indicate the clusters’ tangential velocities.

This study provides a unified, clearly structured scheme for identifying and classifying Galactic binary clusters. It indicates that hierarchical star formation may be an important channel of star formation and offers key observational evidence for the formation mechanisms and dynamical evolution of multi-cluster systems,supporting a hierarchical, clustered scenario of star formation across multiple scales.

This work received an positive feedback during peer review, with an anonymous referee noting:“This is a good paper that offers new insights into a topical issue.”

This study was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences, and Xinjiang regional funds.