galactic halo

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Any material in a roughly spherical distribution around a galaxy, and extending out to beyond the visible regions. It can refer to a population of old (Population II) stars, including globular clusters, with little or no rotation about the galactic centre; to tenuous, highly ionized, high-temperature gas (106 K) around a galaxy; or to an extended distribution of invisible dark matter.

Subjects: Astronomy and Astrophysics.

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