Galactic Corona

the aggregate of globular clusters occupying an almost spherical volume that is concentric with the center of the Milky Way System and whose average diameter exceeds the diameter of the Milky Way System. The galactic corona also includes a large number of stars that together with globular clusters form the spherical part of the Milky Way System. In particular, it includes the short-period cepheids found in the globular clusters themselves, for which they serve as distance indicators. The galactic corona is sometimes called the galactic halo. In some instances in astronomical literature, the galactic corona is defined as the extremely rarefied medium, consisting of gas, high-energy electrons, and cosmic rays, that fills the vast ellipsoidal volume surrounding the Milky Way System beyond the limits of the diffusion of its stellar component. Then halo is understood to mean only the stellar part of the galactic corona (including globular clusters). The sources of the formation of the gas corona, the density of which is 10^-28 g/cm³, are believed to be supernovas, whose explosions send off high-energy electrons and cosmic rays, which rise above the galactic plane and tend to diffuse spherically. The rapid expansion of the gas affects the galactic magnetic fields in the galactic corona in which electrons and cosmic-ray particles are traveling at enormous speeds. The deceleration that occurs in the magnetic fields, however, leads to the heavily polarized emission of electrons in the meter radio-frequency region, which is recorded by radio telescopes. An elongated corona has also been found around the galaxy in Andromeda, and it is found in several other galaxies as well.