A massive star-cataloging project has discovered a curious phenomenon in our own Milky Way Galaxy: Multiple, overlapping halos of stars that rotate in opposite directions.
The separate components of the galaxy seem to have different origins, as evidenced by stars with strikingly different chemical composition, researchers with the Sloan Digital Sky Survey (SDSS-II) say.
"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.
According to the scientists, the outer reaches of our galaxy break down as follows. An inner disk of stars, where our Sun lies, is circling the center of the galaxy with a fantastic rotational speed of about 500,000
miles per hour.
Beyond that, an inner halo of stars rotates in the same direction, but at the slower speed of 50,000 miles per hour. The outer halo appears to be spinning at about 100,000 miles per hour in the opposite direction.
Researchers say this inner halo was likely formed from smaller galaxies that originally rotated with the Milky Way. The outer halo could have been formed by smaller galaxies that orbited the Milky Way in the opposite direction, but were ultimately torn apart by its gravitational field.
A paper on the issue is being published in tomorrow's edition of Nature.
The Milky Way Has a Double Halo [SDSS-II press relesase]
(Images: One: The Milky Way in infrared light, as seen from our region of space by the Cosmic Background Explorer (COBE) satellite. Credit: NASA. Two: A diagram depicting the halos and their spin. Credit: SDSS-II, Masashi Chiba, Tohoku University, Japan)
