The large-scale first analysis of star-exclusions changed our understanding of their origin

What are “runaway stars”?

Runaway stars are objects that move freely through space and are not held by the gravity of star clusters. Their speeds can range from a few tens to over 700 km/s; at the higher end they can escape the boundaries of the galaxy.

Why are they studied?

These objects play a key role in the evolution of galaxies:

* They ionize interstellar gas, aiding the formation of new stars outside clusters.
* Through supernova explosions they disperse heavy elements throughout space, possibly even ending up in our own blood.

What was the 1960s hypothesis?

In 1960, when runaway stars were first detected, scientists proposed a main theory:

* Most runaways are produced in binary systems; when one star explodes as a supernova, its companion receives a kick and starts moving rapidly.
This model predicts that such objects should rotate quickly.

An international ESO group conducts new research

Using the European Southern Observatory (ESO), astronomers gathered data on 214 O‑type runaway stars in the Milky Way. The information came from two sources:

1. The IACOB project – observations of OB-class stars.
2. The Gaia cascade – ESA’s astrometric mission, providing precise velocities and motion vectors.

Thus, for each star, its speed, direction, rotational velocity, and presence or absence of a companion are known.

Key findings

* Most runaway O-stars are single and rotate slowly.
* There is an almost complete lack of objects combining high speed with rapid rotation – contradicting the supernova-in-binary hypothesis.
* Only 12 runaway binary systems were found, including one pair with neutron stars or black hole candidates.

What does this mean?

The results confirm that several mechanisms simultaneously produce runaways:

1. Supernova in a binary system – explains some objects, especially those rotating fast.
2. Dynamical ejection – the more common way to achieve the highest speeds; it occurs through gravitational interactions (triples and multi-body encounters) in dense young clusters.

These conclusions significantly refine models of massive-star evolution, star cluster dynamics, and the impact of supernovae on the galactic environment. Future deeper observations are expected to further test the proposed mechanisms.