The Milky Way’s Stolen Stars

Astronomers say some of the galaxy’s most distant stars were actually whisked away from a passing mini-galaxy.

The force of gravity can be described using a number of metaphors: It’s the glue that holds solar systems and galaxies together, the anchor that keeps us on the ground, the slingshot that sends spacecraft deeper into the solar system.

And in some cases, gravity is a thief.

Astronomers at the Harvard-Smithsonian Center for Astrophysics said Wednesday they have discovered that some of the most distant stars in the Milky Way galaxy actually came the Sagittarius dwarf galaxy, one of the dozens of smaller galaxies surrounding ours. When Sagittarius passed by, the Milky Way’s gravitational tides pulled on the galaxy, and made off with five stars.

The five interlopers are among the farthest known stars in the Milky Way, located in a stretch of stars outside the galaxy’s spiral disk. Astronomers determined their origin using computer simulations. They simulated the movements of the Sagittarius and Milky Way galaxies over the course of 8 billion years. The test showed that over time, Sagittarius lost about one-third of its stars. The stars, the scientists say, were whisked away by the Milky Way and settled into streams at the galaxy’s edge. The observed position and velocities of the stolen stars also suggests they came from Sagittarius, they say.

This kind of cosmic theft may be common in the universe, particularly in neighborhoods of closely orbiting galaxies. In 2011, scientists using a ground-based telescope in Chile to study the Large Magellanic Cloud, another satellite galaxy of the Milky Way, discovered that more than 5 percent of the stars inside the cloud were spinning in the wrong direction. What’s more, their chemical composition, particularly the ratios of elements like iron and calcium, didn’t match that of other stars inside the cloud. Instead, they looked a lot like stars in the Small Magellanic Cloud, its sister galaxy. The stars inside the larger galaxy, they concluded, must have been torn away, thanks to gravitational tides.

Galaxies were once considered “island universes,” but maybe, just like humans, “no galaxy is an island” after all.