The four main components of a galaxy are the disk, the nuclear bulge, the halo and the galactic corona.
The halo is the outermost part of the spiral galaxy, nearly spherical and lying beyond the spiral.
Halo is the component of our galaxy that contains only a small amount of interstellar medium.
Interstellar medium refers to the gas and dust that lies in between the stars in the Milky Way Galaxy. By observing in high-energy wavelengths such as X rays and long wavelengths of light such as radio waves then we can see through the interstellar medium.