New method developed to identify runaway stars ejected from star clusters

The study, led by Alonso Herrera, CATA Affiliated Investigator, provides new tools for studying the dynamics of star clusters and gaining a better understanding of how stars evolve in our galaxy.

An innovative method for identifying runaway stars—stars that are violently ejected from star clusters—was developed by Alonso Herrera Urquieta, a Affiliated investigator at the Center for Astrophysics and Associated Technologies (CATA) (ANID Basal Center) and a doctoral student in the Department of Astronomy at the Universidad de Concepción (UdeC).

The study, published in May 2025 in the prestigious international journal *Astronomy & Astrophysics* (A&A), opens a new window into understanding the internal dynamics of star clusters and the evolution of stars in our galaxy.

Cosmic chaos: when three stars collide

In star clusters—groups of hundreds or thousands of stars that form together—gravitational interactions are common and can be intense and chaotic. In particular, one of the most common mechanisms occurs when three stars interact: a binary system and a single star. This type of encounter typically destabilizes the system, causing one star to be ejected at high speed, while the other two remain together to form a binary system, which may also be ejected from the cluster.

The stars ejected as a result of this process are known as runaway stars. Unlike stars that slowly leave a cluster due to their natural evolution—a process known as evaporation—runaway stars are ejected at unusually high speeds as a result of three-body dynamic interactions, which is why they are called fast escapees (stars that rapidly escape the cluster) and stand out clearly in observational data.

Although this phenomenon has been studied for decades from a theoretical perspective, identifying both stars ejected from the same event through observation has been a challenge until now.

A systematic method for tracking ejected stars

The research led by Herrera proposes a new observational method that makes it possible to simultaneously search for both the solitary star and the binary system that were ejected together from a star cluster. To do this, the team used high-precision data from the European Gaia satellite (Data Release 3), which allows for the measurement of stellar positions and velocities with an unprecedented level of detail.

To test the method, the open cluster M67—one of the most studied in the Milky Way—was analyzed. Using data from more than 15,000 stars in its vicinity, nearly 120 million possible combinations were evaluated, applying strict physical criteria based on the conservation of angular momentum, the direction of motion, and the ejection time.

The result was the identification of a candidate pair—a fast star and a slower binary system—whose physical and dynamical properties suggest that they were ejected from the cluster together. Although this may seem like a small number, the finding is consistent with theoretical predictions for the field of view studied.

A key contribution to stellar astronomy

“The main contribution of this work is that, for the first time, it proposes a systematic method for identifying both products of a three-body interaction outside the cluster,” explains Herrera. Previous studies had focused mainly on simulations or on detecting individual fast-moving stars, without identifying the entire pair.

This breakthrough not only allows for a better understanding of the internal dynamics of star clusters, but could also help us understand the formation of compact binary systems—and even detect more extreme objects, such as black holes, capable of ejecting stars at even higher speeds.

The study involved researchers from various national and international institutions, including the University of Concepción, the American Museum of Natural History (U.S.), the University of Toronto (Canada), the Max Planck Institute for Astrophysics (Germany), the University of Amsterdam (Netherlands), and the University of Wisconsin–Madison (U.S.), among others.

Next steps: more clusters and open science

During his first year of doctoral studies, the researcher is expanding his analysis to a much larger number of open clusters, with the aim of conducting population studies and comparing how variables such as mass, age, and cluster size influence the formation of runaway stars.

In addition, the code is being optimized so that it can run on mid-range computers, making it easier for other researchers, students, and data analysis enthusiasts to use.

At the same time, the team is developing an interactive web platform, in collaboration with programmer Gonzalo Díaz, which will serve as a data repository for more than 2,000 star clusters. This tool aims to make astronomy more accessible to both astronomy and data science students and the general public, promoting science outreach and, in the future, citizen science.