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The Dark Halo of the Milky Way |
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Most of the matter in the Milky Way is invisible to astronomers. Precise numbers are elusive, but it appears that the dark component is 20 times as massive as the visible disk of stars and gas. This dark matter is distributed in space differently than the stars, forming a vast, diffuse halo, more spherical than disklike, which occupies more than 1000 times the volume of the disk of stars. The composition of this dark halo is unknown, but it may comprise a mixture of ancient, degenerate dwarf stars and exotic, hypothetical elementary particles. |
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| Halo The galactic disk is surrounded by a spheroid halo of old stars and globular clusters, whose 90% lie within 100,000 light-years,[22] suggesting a stellar halo diameter of 200,000 light-years. However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years away from the galactic center. | ||
| The halo is used in Hellenistic representations of gods and goddesses and those associated with them. Similarly, in Christianity, halos around the head of a figure mark it as divine or saintly. In the third and fourth centuries, the halo or nimbus (Latin: ‘cloud’ or ‘mist’) was used only for Christ and the lamb. In the fifth century and after, it was extended to the Virgin Mary, angels, and saints. By the eighth century, square halos were used to designate donors, bishops, and popes. | ||
The halo of our Galaxy is the large roughly spherical distribution of stars of our Galaxy. The stars (and globular clusters) are thought to have formed in the early stages of the formation of the Milky Way (and are referred to as Population II stars) and typically will therefore be stellar remnants, low mass stars, or other low mass objects (e.g., planets or brown dwarfs). Halo stars are not the first generation of stars, however, because they contain elements heavier than hydrogen and helium, that is, material which has been processed through stars at least once. According to the measurements of the mass of the Milky Way, it is inferred that the bulk of the mass of the Milky Way is Dark Matter contained in the halo of the Galaxy. It is not clear in what form the mass resides, however. Over the years, the halo material has been suggested to be in the form of normal stars, planet-sized objets, brown dwarfs, black holes, neutron stars, white dwarfs, and fundamental particles (e.g., exotic particles such as Weakly Interacting Massive Particles [WIMPs, that may include neutrinos], axions, ...). |
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