1. Our galaxy's double halo Department of Physics National Tsing Hua University G.T. Chen 2007/12/20 http://www.astronomy.com http://www.astronomy.com December 12, 2007

3. News Using 20,000 stars observed by the Sloan Digital Sky Survey, D. Carollo et al. have discovered that the outer Milky Way is a mix of two distinct components rotating in opposite directions --- There are two stellar components in the halo of the Milky Way. Using 20,000 stars observed by the Sloan Digital Sky Survey, D. Carollo et al. have discovered that the outer Milky Way is a mix of two distinct components rotating in opposite directions --- There are two stellar components in the halo of the Milky Way.

4. News By examining the motions and chemical compositions of the stars, they see that the inner and outer halos are quite different. By examining the motions and chemical compositions of the stars, they see that the inner and outer halos are quite different. The inner-halo component dominates the population of halo stars found at distances up to 10~15 kpc. The inner-halo component dominates the population of halo stars found at distances up to 10~15 kpc. The outer-halo component dominates in the regions beyond 15~20 kpc. The outer-halo component dominates in the regions beyond 15~20 kpc.

7. News The inner halo formed first from the collision of smaller but massive galaxies that rotated with the Galaxy. The inner halo formed first from the collision of smaller but massive galaxies that rotated with the Galaxy. The outer halo formed later from small galaxies orbiting the Milky Way in the reverse direction. The outer halo formed later from small galaxies orbiting the Milky Way in the reverse direction.  These were split apart by the Milky Way's gravitational forces, dispersing their stars into the halo.

8. References http://www.astronomy.com http://www.astronomy.com http://www.astronomy.com December 12, 2007 December 12, 2007 Nature 450, 1020 - 1025 (13 Dec 2007) Nature 450, 1020 - 1025 (13 Dec 2007)

9. >>Thank you >Thank you<<

10. News a disk of stars orbiting around the center of the Galaxy at an astounding 500,000 miles per hour. a disk of stars orbiting around the center of the Galaxy at an astounding 500,000 miles per hour. The inner halo, located well outside the disk, rotates in the same direction, but more slowly, at 50,000 miles per hour. The inner halo, located well outside the disk, rotates in the same direction, but more slowly, at 50,000 miles per hour. The outer halo, the most remote of these components, spins in the opposite direction, at about 100,000 miles per hour. The outer halo, the most remote of these components, spins in the opposite direction, at about 100,000 miles per hour.

11. News inner-halo stars contain three times more heavy atoms than outer-halo stars inner-halo stars contain three times more heavy atoms than outer-halo stars [Fe/H] inner =-1.6,[Fe/H] outer =-2.2 [Fe/H] inner =-1.6,[Fe/H] outer =-2.2

12. News The patterns of chemical elements provide clues to the properties of stars that formed in the first billion years after the Big Bang. The patterns of chemical elements provide clues to the properties of stars that formed in the first billion years after the Big Bang.