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Our Milky Way galaxy never sits still: it rotates and wobbles. And now, data from the European Space Agency's Gaia space telescope reveal that our galaxy also has a giant wave rippling outwards from its center.

We've known for about a hundred years that the galaxy's stars rotate around its center, and Gaia has Since the 1950s, we've known that the Milky Way's disk is warped. Then, in 2020, Gaia discovered that this disk wobbles over time, similarly to the motion of a spinning top.

And now it has become clear that a great wave stirs the motion of stars in our galaxy over distances of tens of thousands of light-years from the sun. Like a rock thrown into a pond, making waves ripple outwards, this galactic wave of stars spans a large portion of the

The unexpected galactic ripple is illustrated in this figure above. Here the positions of thousands of bright stars are shown in red and blue, overlaid on .

In the left image, we look at our galaxy from "above." On the right, we see across a vertical slice of the galaxy and look at the wave side-on. This perspective reveals that the "left" side of the galaxy curves upward and the "right" side curves downward (this is the warp of the disk). The newly discovered wave is indicated in red and blue: in red areas, the stars lie above, and in blue areas the stars lie below the warped disk of the galaxy.

Even if no spacecraft can travel beyond our galaxy, Gaia's uniquely accurate vision—in all three spatial directions (3D) plus three velocities (moving towards and away from us, and across the sky)—is enabling scientists to make these top-down and edge-on maps.

From these, we can see that the wave stretches over a huge portion of the galactic disk, affecting stars around at least 30–65 thousand light-years away from the center of the galaxy (for comparison, the Milky Way is around 100 thousand light-years across).

"What makes this even more compelling is our ability, thanks to Gaia, to also measure the motions of stars within the galactic disk," says Eloisa Poggio, who is an astronomer at the Istituto Nazionale di Astrofisica (INAF) in Italy, and led the team of scientists that discovered the wave.

"The intriguing part is not only the visual appearance of the wave structure in 3D space, but also its wave-like behavior when we analyze the motions of the stars within it."

The motions of the stars are made visible with the white arrows in the edge-on image of the Milky Way above. What can be noticed, is that the wave pattern of the vertical motions (represented by the arrows) is slightly shifted horizontally relative to the wave pattern formed by the star's vertical positions (indicated by the red/blue colors).

"This observed behavior is consistent with what we would expect from a wave," Poggio explains.

Think of a "wave" performed by a crowd in a stadium. Given that galactic timescales are much longer than ours, imagine seeing this stadium wave frozen in time, much like how we observe the Milky Way. Some individuals would be standing upright, some would have just sat down (as the wave passed), and others would be preparing to stand up (as the wave approaches them).

In this analogy, the people standing upright correspond to the regions colored in red in our face-on and edge-on maps. And, if we consider motions, the individuals with the largest positive vertical motions (represented by the largest white arrows pointing upwards) are those who are just starting to stand up, ahead of the incoming wave.

Poggio and her colleagues were able to track down this surprising motion by studying the detailed positions and movements of young giant stars and Cepheid stars. These are types of stars that vary in brightness in a predictable way, which can be seen by telescopes like Gaia over large distances. The findings are in the journal Astronomy & Astrophysics.

Because young giant stars and Cepheids move with the wave, the scientists think that gas in the disk might also be taking part in this large-scale ripple. It is possible that young stars retain the memory of the wave information from the gas itself, from which they were born.

Scientists do not know the origin of these galactic shakes. , but they need to investigate further.

The great wave could also be related to a smaller-scale rippling motion seen 500 light-years from the sun and extending over 9,000 light-years, .

"However, the Radcliffe Wave is a much smaller filament, and located in a different portion of the galaxy's disk compared to the wave studied in our work (much closer to the sun than the great wave). The two waves may or may not be related. That's why we would like to do more research," Poggio adds.

"The upcoming will include even better positions and motions for Milky Way stars, including variable stars like Cepheids. This will help scientists to make even better maps, and thereby advance our understanding of these characteristic features in our home galaxy," says Johannes Sahlmann, ESA's Gaia Project Scientist.