A roughly spherical region surrounding a galaxy, containing old stars, globular clusters, and dark matter. The halo extends far beyond the visible disk and contains most of a galaxy's total mass.
From Greek 'galaktikos' (milky, relating to galaxies) + 'halos' (disk of light around the sun or moon). The astronomical usage emerged in the mid-20th century when astronomers realized galaxies had extended, invisible components.
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