Transcript: Most galaxies are made of stars, gas, and dust plus the ubiquitous dark matter. Elliptical and spiral galaxies contain different proportions of these materials, less dust and less gas for elliptical galaxies than spiral galaxies, but they are basically made of the same thing. The morphology of the galaxies is determined by their stellar orbits and the evolution of stellar populations. Their spectra are simply the sum of the spectra of billions of individual stars. However, certain galaxies have violent phenomena taking place in their centers. This can manifest in a number of ways, by a fierce starburst, by active x-ray emission, by non-thermal radio emission, or by gas moving with very high velocities near the nucleus. Collectively such galaxies are called active galaxies or AGN for active galactic nuclei.