What is the major observational difference between a Seyfert galaxy and a normal spiral galaxy?
Seyfert galaxies have bright active nuclei.
A Seyfert galaxy is a spiral galaxy with an active nucleus. An active galactic nucleus occurs when the supermassive black hole at the galactic centre has an accretion disc of dust and gas falling into the black hole. Material falling into the accretion disc gets heated and emits large amounts of electromagnetic radiation.
Type I Seyfert galaxies are very bright sources of visible and ultraviolet light and x-rays.
Type II Seyfert galaxies are very bright sources of infra-red and visible light.
Type II Seyfert galaxies evolve into Type I Seyfert galaxies.
Seyfert galaxies make up about 10% of all galaxies.