The Milky Way and Andromeda are on a collision course and they will smash together in the distant future. Now the European Space Agency’s telescope Gaia has refined the measurements of Andromeda’s speed and the merger is expected to begin in 4.5 billion years, about 600 million years later than previously expected.

Gaia is taking detailed observations of billions of stars in the Milky Way to measure their position and speed, but its instruments are so sophisticated that it can track stars even in other galaxies. Researchers have used the data from the mission’s second data release to study the movement of Andromeda and another close-by galaxy, Triangulum. The findings are reported in the Astrophysical Journal.

Triangulum and Andromeda, also known as M31 and M33, are two of the main galaxies in the Local Group of galaxies. Together with the Milky Way, they make up most of the Local Group’s mass. Armed with the data from Gaia, researchers were able to estimate that the first interaction between our galaxy and Andromeda will be more of a glancing hit than a head-on collision.

By tracking massive bright stars, the astronomers were also able to work out the spin of these objects, the rate at which they rotate on their own axis. This is a measurement that researchers have been working on refining for a century.

“For the first time, we’ve measured how M31 and M33 rotate on the sky. Astronomers used to see galaxies as clustered worlds that couldn’t possibly be separate ‘islands’, but we now know otherwise," lead author Roeland van der Marel, from the Space Telescope Science Institute, said in a statement. “It has taken 100 years and Gaia to finally measure the true, tiny, rotation rate of our nearest large galactic neighbour, M31. This will help us to understand more about the nature of galaxies.”

Future motions of the Milky Way, Andromeda and Triangulum galaxies. E. Patel, G. Besla (University of Arizona), R. van der Marel (STScI); Images: ESA (Milky Way); ESA/Gaia/DPAC (M31, M33)

The observations also showed how Triangulum is moving around Andromeda. In the past it has not been clear if M33 had already performed a flyby of M31, or it was just starting to fall in. Now, astronomers have worked out that Triangulum’s velocity can only be explained if this is the first time it’s coming close to Andromeda.

“This finding is crucial to our understanding of how galaxies evolve and interact,” added Timo Prusti, ESA Gaia Project Scientist. “We see unusual features in both M31 and M33, such as warped streams and tails of gas and stars. If the galaxies haven’t come together before, these can’t have been created by the forces felt during a merger. Perhaps they formed via interactions with other galaxies, or by gas dynamics within the galaxies themselves.”