The Milky Way galaxy is far from static.
It spins, it wobbles, and now, thanks to the European Space Agency’s Gaia space telescope, we know it also carries a massive wave rippling across its disk.
This newly discovered “great wave” stretches tens of thousands of light-years and stirs the motion of stars in our galaxy much like ripples spreading across a pond.
For about a century, astronomers have known that stars rotate around the center of the Milky Way.
In the 1950s, they learned that the galaxy’s disk is warped. Then, in 2020, Gaia revealed that this warped disk also wobbles over time, behaving somewhat like a spinning top. Now the Gaia data has gone further, showing that a large-scale wave is moving through the galaxy’s outer regions.
Maps created from Gaia’s observations show thousands of bright stars positioned in ways that reveal the wave. Viewed from above, the Milky Way’s disk looks uneven, with one side curving upward and the other downward.
This curvature, or warp, was already known. But when viewed from the side, Gaia’s maps reveal alternating regions of stars that lie slightly above or below the disk. These regions, marked in red and blue, outline the newly identified wave.
The wave is not just a static shape—it behaves dynamically. By measuring the motions of stars, Gaia also captured how they move in sync with the wave.
White arrows on the star maps indicate stars rising or falling relative to the galactic plane. Interestingly, the wave of motions is slightly offset from the wave of positions, exactly what scientists would expect if this were a true wave phenomenon.
“It’s not only the appearance of the wave that is remarkable, but also the way stars move within it,” said Eloisa Poggio, an astronomer at the Italian National Institute for Astrophysics (INAF), who led the study published in Astronomy & Astrophysics.
She compares the effect to a stadium wave. At any moment, some people are already standing (like the red-colored regions of stars), while others are just beginning to rise (like the stars with upward-pointing motion arrows). The Gaia data gives astronomers a frozen snapshot of this galactic-scale version of a wave in motion.
The team traced the wave by examining young giant stars and Cepheid stars, a type of variable star whose brightness changes in predictable cycles. Because Cepheids can be seen over vast distances, they act as excellent tracers of large-scale galactic motion.
The results suggest that not only stars, but also gas in the Milky Way’s disk, may be taking part in this ripple, and that young stars may carry the “memory” of the wave inherited from the gas clouds in which they formed.
The origins of the wave remain a mystery. One possible explanation is that the Milky Way collided with a smaller dwarf galaxy in the past, setting off ripples that are still visible today.
Another possibility is that the wave is somehow linked to the Radcliffe Wave, a smaller filament of stars and gas about 500 light-years from the sun and stretching across 9,000 light-years. However, the Radcliffe Wave is closer to home and shaped differently, so the two phenomena may or may not be related.
Future data releases from Gaia will help answer these questions. The mission’s upcoming fourth release will provide even more precise positions and motions for stars, including Cepheids, allowing astronomers to refine their maps and deepen their understanding of the Milky Way’s complex structure.
For now, the discovery of this great wave is a reminder that even our own galaxy is restless, dynamic, and full of surprises—shaped by ancient events that continue to ripple across space and time.
