National Geographic News
An illustration of the Milky Way eating a smaller galaxy.

A simulation of the Aquarius stream (shown in pink) in the Milky Way.

Illustration courtesy Arman Khalatyan, AIP

Andrew Fazekas

for National Geographic News

Published February 7, 2011

A new stream of stars has been found running through the constellation Aquarius, and astronomers think the group is all that's left of a smaller galaxy that was recently gobbled up by our home galaxy.

The Milky Way hosts more than a dozen known stellar streams, the remnants of satellite galaxies that were gravitationally torn apart and consumed. Most of the other streams loop around the plane of our disk-shaped galaxy, like octopus tentacles grasping a dinner plate.

(See pictures: "New Proof Spiral Galaxies Eat, Digest Dwarfs.")

But the newfound Aquarius stream is unique because it is embedded within the galactic plane.

Aquarius is the closest stellar stream to Earth yet found, stretching from 1,500 light-years to 30,000 light-years away, in the direction of its namesake constellation. It's also the youngest known stellar stream, since it likely formed when a dwarf galaxy was ripped apart 700 million years ago—a mere blink of the eye in cosmic terms.

(Related: "Dark-Matter Galaxy Detected: Hidden Dwarf Lurks Nearby?")

"The stars themselves are quite old, though—ten billion years," said study leader Mary Williams, a postdoctoral researcher at the Astrophysical Institute Potsdam in Germany.

"So it is an old thing that has recently been eaten. Sort of like an old, moldy snack, I guess."

Stellar Immigrants Have Odd Orbits

Williams and her team found the elusive stellar stream as part of a project called the RAdial Velocity Experiment, or RAVE. Using a wide-field telescope at Siding Spring Observatory in Australia, the RAVE project has so far measured the movements of almost half a million stars in our galaxy.

"We now have enough stars to start doing some really interesting science, such as finding the Aquarius stream," Williams said.

The team can tell the Aquarius stream's stars didn't arise in the Milky Way because of the unusual way they move around the galactic center. (Related: "Mysterious Structures Balloon From Milky Way's Core.")

"We found a group of stars that have an aberrant ... motion through space," Williams said. Most stars orbit around the galactic center, the same way the planets orbit the sun. But the alien stars move together in a path that traces out a pattern similar to a rose with many petals.

"We are detecting the stream [now] as it traces the tip of one of the petals," Williams said. "The stars are turning around in their orbits from going outward to inward, toward the galaxy's center."

(Read about a planet found around a star in a stellar stream.)

By studying the Aquarius stream and other remnants of the Milky Way's past meals, scientists can understand not only the origins of our galaxy, but also its future.

"Our current understanding of galaxy formation does mean that our galaxy grew by merging with and eating others," Williams said. "Eventually it'll merge with the other big boy in the local group, the Andromeda Galaxy. But that's a long time off yet"—in about five billion years, astronomers predict.

The newfound Aquarius stream is described in a paper published in the February 20 issue of the Astrophysical Journal.

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