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In astronomy, a nearly spherical volume of thinly scattered stars, globular clusters, and tenuous gas observed surrounding spiral galaxies. It may extend far beyond the disk and contain most of the galaxy's mass. The halo of the Milky Way Galaxy is thought to have a radius greater than 100,000 light-years and to be predominantly composed of dark matter.
This entry comes from Encyclopædia Britannica Concise. For the full entry on galactic halo, visit Britannica.com.
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