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The Milky Way
The Galaxy's bright stellar disk is embedded in a faint, thicker disk of old stars. This disk, which has a thickness about 3 times that of the thin disk, may have been the original structure from which the thin disk condensed, or it could have been thickened by a collision with a smaller galaxy ten billion years ago.
| Schematic of Milky Way showing the dark matter halo (gray), globular clusters (red circles), the thick disk (purple), the stellar disk (white), the stellar bulge (red-orange), and the central black hole (black dot). The stellar disk is about 100,000 light years in diameter. The dark halo extends to a diameter of at least 600,000 light years. (Illustration: CXC/M.Weiss) |
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Surrounding the thick Galactic disk is an extremely faint halo that contains the oldest stars in the Galaxy. These stars are located in globular clusters, dense swarms of about 100,000 stars. The Galactic halo is dominated by dark matter, a still mysterious form of matter that cannot be seen with any type of telescope, but is detected by its gravitational effects. Studies of the motions of stars and gas in the Milky Way indicate that the mass of the dark matter halo is about twenty times greater than the mass of all the stars in the galaxy.
More on Dark Matter
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