Galactic Disk :
It is the thinnest part of a spiral galaxy. The disk surrounds the nuclear bulge and contains a mixture of old and young stars, gas, and dust. In the case of the Milky Way, it extends out some 50,000 light-years from the Galactic center but is only about 1,000 light-years thick. The dust in the disk (a few hundred light-years thick) creates the dark ribbon that runs the length of the Milky Way and limits the view of our own Galaxy (see dust lanes).
Galactic nucleus :
It is the core of a spiral galaxy. In the case of Seyfert (active) galaxies, the nucleus is extremely luminous in the radio wavelengths.
Galactic Halo :
It is large (50,000 light-year radius) sphere of old stars surrounding the galaxy.