Galactic Halo

a collection of stars, gas and dust, nearly spherical in shape, which has the same center as the galaxy. It contains population II stars and creates a lot of the background radio emissions.

Taken from Dictionary of Science
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The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:

• the galactic spheroid (stars)

• the galactic corona (hot gas, i.e. a plasma)

• the dark matter halo

The distinction between the halo and the main body of the galaxy is clearest in spiral galaxies, where the spherical shape contrasts with the flat disc. In an elliptical galaxy, there is no sharp transition between the body of the galaxy and the halo.

Taken from Wikipedia