Stars on the Run

A substantial number of O and B stars say goodbye to their birth clusters and spend their lives zooming through space alone. New research uses the most recent Gaia data to investigate the properties of these runaway stars.

Massive Stars Far from Home

The Trapezium cluster

OB stars are typically born in clusters, like the Trapezium Cluster at the heart of the Orion Nebula, shown here. [ESO/IDA/Danish 1.5 m/R.Gendler, J.-E. Ovaldsen, and A. Hornstrup; CC BY 4.0]

The majority of the most massive stars in our galaxy — those with spectral types O and B, also called OB stars — live and die within their natal star clusters, never traveling far from where they were born during their brief, luminous lives. But roughly 20–30% of OB stars lead significantly more adventurous lives. So-called runaway stars careen through space on a solo adventure or with a single companion, traveling light-years from their birthplaces.

Stars can be ejected from their birth clusters by dynamical interactions or supernova explosions. Dynamical ejection happens when stars in a crowded cluster pass too close to one another, slingshotting one or more stars into space. A star that explodes as a supernova can expel its close binary companion from the cluster. In rare cases, the exploding star and its companion can escape the cluster together. It’s not yet known which process creates more runaway stars or how the two populations of runaways differ.

Map of the Small Magellanic Cloud with stellar proper motion vectors for potential runaway stars

Vectors showing the motion across the sky of the stars in the authors’ sample. Eclipsing binaries (EB) and double-lined spectroscopic binaries (SB2) were ejected dynamically. High-mass X-ray binaries (HMXB) have experienced a supernova. Click to enlarge. [Phillips et al. 2024]

A Search for Runaway Stars

Diving into this question, a team led by Grant Phillips (University of Michigan) investigated more than 300 potential runaways in a nearby dwarf galaxy, the Small Magellanic Cloud. The stars in their sample were chosen for being at least 90 light-years from other OB stars (with the exception of stars in binary systems, which may have escaped their home cluster together).

The binary systems in the sample immediately gave away how they were launched into space: binary systems that don’t contain a neutron star or a black hole can’t have experienced a supernova — those binaries must have been kicked out of their clusters through dynamical interactions. Binary systems that do contain a neutron star or a black hole must have experienced a supernova. As for solo runaway stars, the team’s earlier research suggested that typical OB stars tend to be expelled through dynamical interactions, while stars with emission lines in their spectra due to rapid rotation — OBe stars — tend to be ejected by supernovae.

Population Comparison

Mass and velocity distributions for stars ejected dynamically and via supernovae

Mass and velocity distributions for stars ejected dynamically (left) and via supernovae (right). Click to enlarge. [Phillips et al. 2024]

Using the third release of Gaia spacecraft data, Phillips’s team calculated the velocities of the stars in their sample. They found that the velocities of the single OBe stars overlapped well with those of the post-supernova binary systems, although the OBe stars’ velocities extended to higher values. While this could be an effect of small-number statistics, it could also be due to some stars reaching high velocities through a two-step process: first, dynamical ejection as a member of a binary system; then, an extra velocity kick when the binary companion goes supernova.

The velocities of typical OB stars are similar to the velocities of binary systems ejected dynamically, supporting the hypothesis that these stars are also ejected this way. Intriguingly, the velocity distribution for typical OB stars may have two peaks. The authors speculate that the peaks could point to two groups of dynamical interactions: stars ejected by just a single other star versus stars ejected by a binary system.

Underpinning these findings is the rich, precise dataset from Gaia, which allowed the team to identify systematic offsets from their previous work based on earlier data releases from the spacecraft. Hopefully, future data releases can give even more clues to the origins of these runaway stars!


“Runaway OB Stars in the Small Magellanic Cloud. III. Updated Kinematics and Insights into Dynamical versus Supernova Ejections,” Grant D. Phillips et al 2024 ApJ 966 243. doi:10.3847/1538-4357/ad3909