You Can Actually See the Milky Way’s Wave When You Map Its Stars

Spiral galaxies are one of the most commonly known types of galaxy.  Most people think of them as large round disks, and know that our Milky Way is counted among their number.  What most people don’t realize is that many spiral galaxies have a type of warping effect that, when you look at them edge on, can make it seem like they are forming a wave.  Now scientists, led by Xinlun Chen at the University of Virginia, have studied millions of stars in the Milky Way and begun to develop a picture of a “wave” passing through our own galaxy.

Since humans are not currently able to view the Milky Way in an edge-on orientation, they must resort to more brute force methods to develop models about the what, if any, wave our galaxy has.  Luckily, scientists now have the tools to do so, in the form of the Sloan Digital Sky Survey and ESA’s Gaia satellite.

The “Integral Galaxy” is one of the spiral galaxies with the most visible warp. Credit: DECaLS

The method the team used was to try to identify and track the motions of as many stars as possible.  To do this, they used the Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectrograph, which is part of the SDSS.  This preliminary data allowed them to look at both the chemical compositions as well as the motions of hundreds of thousands of stars.  While this motion data was helpful in starting to form the picture of the Milky Way’s wave, it was not sufficient to complete it.

Additional data was needed, which was delivered by Gaia – detailed distances to those hundreds of thousands of stars.  Combining the distance data with the motion data allowed the team to build a complete 3D map of the Milky way, or at least as good an approximation of one as can be built from within its confines.


SDSS YouTube video showing the save in the galaxy.
Credit: SDSS YouTube Channel

What that map is able to show us is the “wave” that is present in so many spiral galaxies is also present in our own, though it is not as prevalent as elsewhere.  Another discovery gleaned from this data set was that the wave itself migrates throughout the galaxy, circumnavigating the galaxy approximately once every 440 million years.

Many factors could have caused such a long-lived and continually propagating wave.  But the team conjectures that a theorized collision with a previous satellite galaxy about 3 billion years ago caused the original ripple. It has then kept itself going long after the initiating force had dissipated, and it will continue to do so for a long time to come.  Maybe it will be long enough for us to send a probe outside the galaxy to be able to observe the Milky Way’s own wave from an edge-on vantage point.

Lead Image credit: Adrian Price-Whelan / 2mass

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