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Galaxy HaloA galaxy halo is a feature of most galaxies. In a spiral galaxy, the halo is a roughly spherical distribution of Population II stars, extending to large distances from the galactic plane. Halo stars are very old, and have a significantly lower metallicity than galactic disk stars. In an elliptical galaxy, there is no sharp transition between the body of the galaxy and the halo. Galactic haloes have been suggested as possible locations of substantial amounts of dark matter, in the form of Massive Astrophysical Compact Halo Objects (MACHOs) - planets or brown dwarfs, which emit very little radiation but the total population of which could weigh a significant amount. Searches for these objects by looking for microlensing events have so far suggested that there is not a large population of these objects.
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