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Date: January 19, 2004

UVic Astronomer Discovers Stellar Case of "Cannibalism"

It’s a galaxy-eat-galaxy universe out there and there’s evidence right in our own galactic backyard, according to new research by UVic astrophysicist Dr. Julio Navarro and colleagues in the Netherlands and Australia.

In a paper published this week in The Astrophysical Journal Letters, the team concludes that the star Arcturus—the third brightest star in the night sky—is an alien star from another, smaller galaxy that was swallowed up about 10 billion years ago by our own Milky Way galaxy.

The new finding challenges the traditional theory of galaxy formation, which says that they develop in isolation following the collapse of a giant cloud of gas and dust. “This may be too simplistic,” says Navarro. “Instead, the formation of all galaxies may be punctuated by distinct merger events in which smaller galaxies are disrupted, trapped and swallowed.”

When galaxies merge, their stars don’t collide—they’re far too small—but retain for a long time traces of their previous identity in the form of similar dynamic properties.

Using sophisticated computer simulations, Navarro and his colleagues argue that the peculiar movements of Arcturus and several other associated stars indicate they didn’t originate in the Milky Way. “The main clue is that they follow paths that differ strongly from that of our sun, which, like the majority of stars in the Milky Way, moves on a roughly circular orbit around the centre of the galaxy,” says Navarro.

It’s possible that the Milky Way acquired a large fraction of its stars through such intergalactic cannibalism. In fact, there’s evidence that, even now, the Milky Way is busy swallowing up the Sagittarius dwarf galaxy, which in a few hundred million years, will have been disrupted beyond recognition.

Arcturus is a mere 36 light years away from Earth—very close in astronomical terms. In B.C., it is currently visible in the early morning eastern sky as the dominant star in the constellation of Bootes—the Herdsman that chases across the sky after the Great Bear, Ursa Major.

“It’s oddly gratifying to think of stars visible to the naked eye, such as Arcturus, as silent night sky witnesses of the eventful formation history of our Milky Way,” says Navarro, a theoretical cosmologist who studies the evolution, structure and dynamics of galaxies. His work was featured in a National Geographic article, “Galaxy Hunters,” in February 2003.

Media contacts:
Dr. Julio Navarro (Physics & Astronomy) at (250) 721-6644 or jfn@uvic.ca
Valerie Shore (UVic Communications) at (250) 721-7641 or vshore@uvic.ca

UVic media releases and other resources for journalists are available on the World Wide Web at http://communications.uvic.ca/media

(image: fern)