Astronomers using the European Space Agency's Gaia telescope have discovered a colossal wave propagating through the Milky Way galaxy, affecting stars tens of thousands of light-years away. This undulating motion, visible in the galaxy's disc, resembles ripples in a pond and involves both stellar positions and movements. The origin of this phenomenon remains unknown, though it may stem from a past galactic collision.
The Milky Way, our home galaxy spanning about 100,000 light-years, is not a static structure but one in constant motion. For decades, scientists have observed its rotation around the core and a warping of its disc, first noted in the 1950s. More recently, in 2020, Gaia data revealed that this warped disc oscillates like a spinning top.
Now, a new analysis led by Eloisa Poggio, an astronomer at Italy's Istituto Nazionale di Astrofisica, has identified an enormous wave traveling outward from the galactic center. This 'great wave' influences stars located 30,000 to 65,000 light-years from the center, spanning vast outer regions of the disc.
Gaia's precise 3D measurements of stellar positions and motions—tracking movement toward and away from Earth, as well as across the sky—enabled the creation of detailed top-down and edge-on maps. In these views, red regions show stars above the warped plane, while blue indicates those below, illustrating the wave's pattern.
"What makes this even more compelling is our ability, thanks to Gaia, to also measure the motions of stars within the galactic disc," says Poggio.
The stars' vertical motions, depicted by white arrows in Gaia's visualizations, shift sideways relative to their positions, confirming wave-like behavior. Poggio compares it to a stadium crowd wave: some stars are at the crest (standing upright, in red), while others rise just ahead (longest upward arrows).
The team studied young giant stars and Cepheids, which vary predictably in brightness, allowing observation over large distances. These stars move with the wave, suggesting interstellar gas may also participate, imprinting a 'memory' in newly formed stars.
The wave's cause is uncertain; a past collision with a dwarf galaxy is one hypothesis. It might connect to the smaller Radcliffe Wave, about 500 light-years from the Sun and 9,000 light-years long, but their relation is unclear.
"The upcoming fourth data release from Gaia will include even better positions and motions for Milky Way stars," notes ESA's Gaia Project Scientist Johannes Sahlmann, promising refined maps and deeper insights into the galaxy's dynamics.