Berkeley 59: an expanding cluster described in the study: “Internal dynamics and structure of Cepheus OB4. The asymmetric expansion of Berkeley 59” of B. Wiesneth (Universität Leipzig/Universidade de Lisboa)
Berkeley 59 is a young stellar clusters, which is experiencing a rapid asymmetric expansion
Stars typically form in groups or stellar clusters containing from hundreds to thousands of members. Within these clusters, stars are not stationary but move along trajectories determined by the system’s overall gravity (that is, the gravitational potential of the cluster) and by their mutual interactions. This results in a complex dynamical evolution of the clusters.
In the early stages of their evolution, the dynamics of a cluster may still retain traces of the motions of the molecular cloud from which it formed. Later on, mainly due to the influence of the most massive stars (with masses greater than about eight solar masses), the residual cloud is dispersed. From that moment, the cluster’s dynamical evolution changes dramatically, leading either to its dissolution or to the formation of a more stable stellar group.
In recent years, the study of the dynamical evolution of clusters and stellar associations has been revolutionized by the European Space Agency Gaia satellite. Gaia has provided extremely precise measurements of stellar positions and proper motions, as well as a vast photometric catalog and spectroscopic data for more than two billion stars — many of them belonging to stellar clusters.
For example, thanks to Gaia’s observations, in 2019 a team of researchers from the California Institute of Technology showed that very young clusters (younger than 5 million years) display clear signs of expansion. Later, in 2024, a study led by astronomers from the University of Bologna confirmed that clusters younger than 30 million years typically go through this expansion phase, while older ones tend to exhibit dynamically stable configurations.
Recently, a team of researchers led by astrophysicist B. Wiesneth (Universität Leipzig and Universidade de Lisboa) analyzed Gaia data for the stellar cluster Berkeley 59, located in the Cepheus OB4 region. The researchers identified 1030 stars belonging to the cluster, allowing them to determine several of its properties with greater accuracy: its distance from Earth (about 3030 light-years), its age (2.9 million years), and its radius (about 15.5 light-years). The analysis also made it possible to study the cluster internal dynamics, revealing that Berkeley 59 is currently undergoing an expansion phase, with an average radial velocity component of about 0.52 km/s. Moreover, the expansion is highly asymmetric, with the stars preferentially moving northward.
The study is described in the article “Internal dynamics and structure of Cepheus OB4. The asymmetric expansion of Berkeley 59”, recently published in Astronomy & Astrophysics, with the collaboration of astrophysicists K. Mužić (Universidade de Lisboa) and V. Almendros Abad (INAF – Osservatorio Astronomico di Palermo).
The cover figure (click here to view the full image) shows the proper motion vectors — that is, the motion of stars on the sky — of stars in Berkeley 59, plotted over an image of the region obtained from the DSS survey. The black dots indicate the positions of the stars, while the direction of the lines shows their motion. The concentric circles mark radii of 10, 20, 30, and 40 arcminutes from the cluster center. The proper motion vectors are color-coded according to the angle between the line connecting each star to the center (white star) and its proper motion vector: purple hues indicate motion away from the center, green hues indicate motion toward the center, and white bars mark motion perpendicular to the connecting line. White-colored stars may move in opposite directions with respect to one another.
Mario Giuseppe Guarcello
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