The Milky Way Stole Some of Its Outer Stars from Another Galaxy

The Milky Way is packed with an estimated 400 billion stars, most of which are homegrown. But despite this abundance of native star stuff, our galaxy is not above pilfering a few choice stellar gems off of other galaxies that stray too close to its gravitational pull.

According to new research forthcoming in the The Astrophysical Journal, this scenario explains the origins of some of the most far-flung stars in the Milky Way, located a full 300,000 light years from Earth.

The study was led by Marion Dierickx, a graduate student at the Harvard-Smithsonian Center for Astrophysics (CfA), with contributions from her PhD advisor Avi Loeb, a prolific Harvard physicist. The pair simulated the interactive history between our galaxy and the Sagittarius dwarf galaxy, a smaller cluster of star junk that orbits the Milky Way. They found that the positions and velocities of five of those distant stars match models of the orbital passes the galaxies have made with each other over the last eight billion years.

In other words, the Milky Way appears to have been plucking stars from Sagittarius whenever it has come into close contact with this satellite galaxy. As a result, Sagittarius has lost about a third of its original star population and 90 percent of its dark matter to its larger neighbor.

What's more, the study suggests that these objects are part of hidden "streams" of stars, which are stellar tendrils that have been dislodged from neighboring galaxies by the Milky Way. "The star streams that have been mapped so far are like creeks compared to the giant river of stars we predict will be observed eventually," said Dierickx in a statement. "More interlopers from Sagittarius are out there just waiting to be found."

Such rivers of stars are estimated to stretch out about one million light years from the Milky Way's center, and have already been partially detected by the Sloan Digital Sky Survey. Hopefully, future instruments like the Large Synoptic Survey Telescope, due for first light in 2019, will be able to further resolve these dim celestial bridges between galaxies.

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