Most of the stars of the galaxy are found in two spiral arms that extend outward from the center. I stars and are of all ages. Globular clusters of very old stars occur outside the central disk of the galaxy. These stars are Population II stars.
Spiral galaxies are other collections of stars that resemble our galaxy.
The universe apparently consists of widely separated galaxies of stars. A radio telescope is a directional antenna connected to a sensitive radio receiver. Radio waves from space are produced by extremely hot gases, by fast electrons that move in magnetic fields and by atoms and molecules excited to radiate. Especially notable sources are quasars , distant objects that emit both light and radio waves strongly and that may be powered by supermassive black holes at their centers. Cosmic rays are atomic nuclei, mostly protons, that travel through the galaxy at speed close to that of light.