In the early 1960s, Dutch astronomer Adriaan Blaauw observed stars moving at unusually high speeds moving through the Milky Way. These stars, as it turned out, were unbound objects that had been kicked out of the Milky Way and periodically looped back and forth through the disk. Blaauw proposed that these stars originated in binary systems and were ejected when the companion star collapsed and exploded off its outer layers in a supernova. By 2005, even faster runaway stars were observed, leading to the designation "hypervelocity stars."
In January, researchers from institutes across Spain announced the completion of the most extensive observational study to date of runaway massive stars. Using data from the ESA's Gaia Observatory and high-quality spectra from the IACOB Spectroscopic Database, the team analyzed 214 O-type stars, the brightest and most massive class of stars in the galaxy. Their results shed new light on how these stellar objects are ejected into space and their origins. In particular, they show that the majority of runaway stars did not begin as binary companions.
These stellar runaways are of interest to astronomers because of the influence they have on the evolution of galaxies. By escaping their systems of origin, they irradiate gas and dust in the interstellar medium (ISM), eventually seeding it with heavy elements after they go supernova. This, in turn, affects how future stars and planets will form in the ISM. Understanding the origins of runaway massive stars will lead to refined models of stellar evolution, as well as new models on how binary systems, star clusters, and supernovae affect galactic evolution.
Since their discovery, astronomers have wondered how these stars acquire such high velocities and considered two possible scenarios: explosive ejection by supernovae in binary systems or gravitational ejection from close encounters with star clusters. However, the relative contributions of these mechanisms for turning giant stars into runaways in our Milky Way remained unknown. In short, scientists could not say which of the possible scenarios was the most likely and the most common. To shed light on this, the team of Spanish researchers analyzed Gaia and IACOB data to characterize these stars.
Between 2013 and 2025, the Gaia Observatory measured the proper motion, luminosity, temperature, and composition of over 2 billion stars in the Milky Way - a process known as astrometry. This data will be used to create the most precise three-dimensional map of the Milky Way to date, addressing many unanswered questions about the origin, structure, and evolution of our galaxy. The IACOB project, meanwhile, is a long-term observational campaign aimed at providing a comprehensive overview of the physical properties and evolution of massive OB-type stars in the Milky Way.
By combining these two sources of data, the team was able to measure the rotation speed and the point of origin for the largest sample of galactic O-type runaway stars to date. By definition, this term applies to stars that have velocities that often exceed 700 km/s (435 mi/s)—fast enough to escape the Milky Way's gravity. The results reveal that most runaway stars rotate slowly, while those that rotate faster are more likely to be linked to supernova explosions in binary systems. They also found that the highest-velocity stars tend to be single, suggesting that they were ejected from young clusters through gravitational interactions.
The team also identified 12 runaway binary systems, including three X-ray binary sources that contain neutron stars or black holes, and three additional systems that are likely candidates for hosting black holes. "This is the most comprehensive observational study of its kind in the Milky Way," said lead author Mar Carretero-Castrillo, a member of the ICCUB and IEEC, currently at the European Southern Observatory. "By combining information on rotation and binarity, we provide the community with unprecedented constraints on how these runaway stars are formed."
Lastly, the team found that virtually no stars in their study exhibited high velocities and rapid rotation. This was the strongest evidence that multiple mechanisms are responsible for ejecting stars from their systems. Future Gaia data releases and ongoing spectroscopic studies will help astronomers trace these stars to their birthplaces within the Milky Way. This will allow them to confirm which mechanism was responsible and could lead to the discovery of more exotic binary systems, including those that still have gravitationally bound systems of planets.
The study of these systems could shed light on another role they may play in galactic evolution: the distribution of the basic ingredients of life throughout the Milky Way.
Further Reading: UB, Astronomy & Astrophysics