The Milky Way is not hanging alone in space — several dwarf galaxies are hovering nearby, and one of them has been a particular target of study for astronomers. Using a new space telescope, researchers from the University of Michigan have determined that the Small Magellanic Cloud is flying apart at the seams after a collision millions of years in the past.

At a distance of about 200,000 light years, the Small Magellanic Cloud (SMC) is visible from Earth’s southern hemisphere. This dwarf galaxy is still vast compared with the scale of Earth with a diameter of 7,000 light years and several hundred million stars. It’s an irregular galaxy, but astronomers have long suspected that hints of a central bar structure mean it could have been a more ordered “barred spiral galaxy” in the past. While studying stellar motion in the Small Magellanic Cloud, the Michigan team may have discovered why it’s not a spiral galaxy anymore.

The team used the ESA’s Gaia telescope to track 315 stars in the Small Magellanic Cloud. Gaia is designed to image stars repeatedly over the course of several years to build a map of their location and movement. The Small Magellanic Cloud is an entirely self-contained galaxy, so Gaia has the capacity to follow all the significant changes inside it. That makes it a perfect way to study runaway stars — those objects that get flung clear of their homes by powerful events.

There are two main mechanisms that eject stars from a galaxy. A supernova in a binary system can fling the remaining star away and produce a burst of X-rays in the process. A close pass by an object with high gravity can also slingshot a star out of a galaxy. Gaia found evidence of both in the Small Magellanic Cloud. However, it also saw that a region of the SMC known as the Wing is moving away from the main body. However, all those stars are moving in the same direction.

Gurtina Besla, an astronomer at the University of Arizona contributed to the study. In the past, her team modeled a suspected collision between the SMC and the Large Magellanic Cloud, which is twice the size. The group predicted that a near miss would cause the Wing to fly off in a perpendicular direction. Meanwhile, a collision would drag the Wing toward the Large Magellanic Cloud. That’s what Gaia shows — the Wing is separating from the SMC and moving in toward the Large Magellanic Cloud.

The team estimates that the two dwarf galaxies collided within the last several hundred million years, but there’s still a lot to learn. With data from Gaia and other upcoming telescopes, we’ll be able to study this separation in unparalleled detail. We’re lucky that the Small Magellanic Cloud is in our galactic neighborhood.

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