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The Sciences

Caught in the Act: Our Cannibal Galaxy

The Milky Way’s gravitational tides are pulling two long streams of stars from the body of the Sagittarius dwarf galaxy. As our sun (yellow dot) passes through one stream, we get a close-up view of stars born in another galaxy. | David Law/University of Virginia


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Galaxies grow by eating their own, and the Milky Way is no exception. Astronomer Steven Majewski at the University of Virginia has produced a detailed all-sky map that shows, in unprecedented detail, our galaxy shredding and consuming a neighbor just 1/10,000 its size.

Using the Two-Micron All-Sky Survey, Majewski and his colleagues picked out the stars that belonged to the tiny Sagittarius galaxy by their distinctive chemical “fingerprints.” Sagittarius contains more heavy elements than the Milky Way, so its aging stars appear slightly redder.

The researchers sifted through the survey’s vast database of infrared observations and found a series of distinctive ruddy stars stretched out in two great arcs that pass over and under the Milky Way’s disk. Those stellar streamers are about all that is left of the small galaxy.

Every 750 million years, the oval orbit of the Sagittarius galaxy brings it close to ours, where gravitational tides pull it apart like a piece of cosmic taffy. “Each time, many stars find themselves more attracted to the Milky Way than the Sagittarius galaxy,” Majewski says.

Computer models show that previous passes transformed Sagittarius from a dwarf spiral galaxy into a spherical blob. After the latest encounter, most of its remaining stars were no longer bound together, but they have yet to drift away and completely blend into the Milky Way’s halo. By chance, Earth lies inside the stream of stars, giving us a unique perspective on the process.

“While Sagittarius has presented us with a particularly dazzling view of its dance of death, it is likely that similar events have been common in the life of the Milky Way,” Majewski says. Sure enough, an independent team of astronomers led by Rodrigo Ibata of Strasbourg Observatory in France just found another miniature galaxy falling prey to our own. Using a similar infrared-search technique, they spotted the remains of the Canis Major dwarf galaxy wrapped around the Milky Way.

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