Any of the billions of systems of stars and interstellar matter that make up the universe. Galaxies vary considerably in size, composition, structure, and activity, but nearly all are arranged in groups, or clusters, containing from a few galaxies to as many as 10,000. Each is composed of millions to trillions of stars; in many, as in the Milky Way Galaxy, nebulae can be detected. A large fraction of the bright galaxies in the sky are spiral galaxies, with a main disk in which spiral arms wind out from the centre. The arms contain the greatest concentration of a spiral galaxy's interstellar gas and dust, where stars can form. Surrounding the centre (nucleus) is a large, usually nearly spherical nuclear bulge. Outside this and the disk is a sparse, more or less spherical galactic halo. In elliptical galaxies, which vary greatly in size, stars are distributed symmetrically in a spherical or spheroidal shape. Dwarf ellipticals (with only a few million stars) are by far the most common kind of galaxy, though none is conspicuous in the sky. Irregular galaxies, such as the Magellanic Clouds, are relatively rare. Radio galaxies are very strong sources of radio waves. Seyfert galaxies, with extremely bright nuclei, often emit radio waves and may be related to quasars.