Hidden Neighbor

StarDate: November 17, 2009

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Our galaxy, the Milky Way, is so large and heavy that more than two dozen smaller galaxies orbit it. Two of these galaxies are even visible to the unaided eye -- at least to those who live south of the United States.

And astronomers at the University of California at Berkeley say the Milky Way has another galactic companion that no one has ever seen. And it rivals the largest of its known satellites.

The astronomers studied ripples in the gas at the Milky Way's edge. They say the ripples were created by the gravitational pull of an unseen satellite galaxy.

According to their simulations, the galaxy revolves around the Milky Way on a stretched-out orbit. When closest, the galaxy is just 16,000 light-years from the Milky Way's center -- much closer to the center than we are. At present, though, the galaxy is about 300,000 light-years out.

The galaxy is about 10 billion times as massive as the Sun. That's only about one percent as massive as the Milky Way. But it's about equal to the Large Magellanic Cloud, which is the Milky Way's heaviest known satellite.

Despite its heft and its closeness, no one has ever seen this galaxy. The astronomers suspect that it's on the other side of the Milky Way from us, hiding behind thick clouds of gas and dust. In the future, astronomers hope to pinpoint the galaxy's location, so they can finally see what may be the Milky Way's greatest galactic satellite.

Script by Ken Croswell, Copyright 2009

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