The Andromeda Galaxy, also known as M31, is the closest spiral galaxy to the Milky Way and the largest member of the Local Group. Recent findings reveal that these two galaxies, separated by 2.5 million light-years, have already begun a slow-motion collision. While it will take billions of years for them to fully merge into one elliptical galaxy, data from the European Space Agency’s Gaia telescope suggests that the first phase of this galactic encounter has already started, with the Milky Way and Andromeda swapping stars.

The stars in the Milky Way generally orbit its center, with the Sun taking around 220 million years to complete one orbit at a speed of 250 km/s. This is still insufficient to escape the galaxy’s gravitational pull. However, in 2005, scientists at the Harvard-Smithsonian Center for Astrophysics discovered an unusual phenomenon: a star speeding away from the Milky Way’s center at over 700 km/s, enough to break free from the galaxy’s gravity. Stars with this velocity are known as “high-velocity stars” (HVS), and more have since been observed.

One explanation for these runaway stars is the interaction of binary stars with the Milky Way’s central supermassive black hole. When one of the binary stars is captured by the black hole, the other is catapulted out of the galaxy. Another theory suggests that high-velocity stars may result when a companion star in a binary system explodes in a supernova, giving the remaining star a powerful push outward.

Another theory is that some of these high-speed stars might not be native to the Milky Way but could have migrated from Andromeda. To explore this possibility, a team from the Institute of Astrophysics in Karlsruhe, led by Lukas Gulzow, used data from the Gaia mission to study the speeds and trajectories of HVS within the Local Group. The Gaia data allowed them to track nearly 18 million high-velocity stars, which they then compared to a model of the gravitational potential within the Local Group.

Their analysis found that some of these stars’ trajectories likely originated from Andromeda millions of years ago. Likewise, it’s likely that some high-velocity stars from the Milky Way are on a one-way journey to Andromeda. This star exchange indicates that gravitational forces are already at work between the two galaxies, even though they’re still millions of years from merging.

As Andromeda and the Milky Way converge at a speed of 85 km/s, the gravitational influence of the Milky Way accelerates these runaway stars, giving them what’s known as a “hyperbolic excess of speed.” This boost often sends them out on intergalactic journeys, ensuring they eventually leave the Milky Way. However, some may linger if repeated gravitational interactions with the galaxy’s stars slow them down, potentially making them “permanent residents.”

While Andromeda and the Milky Way are still far from their grand union, the ongoing exchange of stars marks the beginning of their future merger. This interaction underscores the immense scale of galactic processes and shows how even at distances of millions of light-years, galaxies can interact, paving the way for an eventual cosmic unification.