By Robert Roy Britt
Senior Science Writer
Our Milky Way Galaxy has two distinct parts in its outer reaches that rotate in opposite directions, astronomers announced today.
The galaxy has a bulbous core where stars are tightly packed and orbiting rather furiously around the central black hole. Then there's the big flat disk with its spiral arms, also orbiting the galactic center somewhat in the manner of a hurricane's spiral bands. We live on one of those arms. Around it all is a halo of stars that don't all behave in such an orderly fashion. That much researchers knew.
Now they find the halo has two parts.
"By examining the motions and chemical makeup of the stars, we can see that the inner and outer halos are quite different beasts and they probably formed in different ways at different times," said Daniela Carollo, a researcher at Italy's Torino Observatory and the Australian National University.
The finding, detailed in the Dec. 12 issue of the journal Nature, is based on 20,000 stars observed as part of the Sloan Digital Sky Survey.
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