Active galaxies: Galaxies that produce much more energy than usual ones, emitting strongly at infrared, ultraviolet and X ray wavelengths. The energy is always generated at the galactic cores. Double-lobed radio source: A galaxy located between two lobes which emit strong radio waves. The lobes are generally larger than the galaxy and are produced by high energy particles ejecting from the galactic core (Fig. 15-9a). Fig. 15-9 (a) Galaxy NGC 4261 looks like a fuzzy blob in an earth-based photograph at visible wavelengths (white), but radio maps reveal radio lobes (oranges). (b) A Hubble Space Telescope image of the core of the galaxy reveals a bright central spot surrounded by a disk. Astronomers assume the disk feeds matter into a central black hole that ejects jets feeding the radio lobes. Seyfert galaxies: Spiral galaxies with unusually bright, tiny cores that fluctuate in brightness over a short time (some ~ 100 seconds) the energy sources at the cores are very small Supermassive black holes theory: Very massive black holes (millions to billions solar mass) exist in the cores of active galaxies. Accretion of in-falling matter into the black hole (Fig. 15-10), ejects powerful jets in both direction along the axis of rotation a double-lobed radio source A black hole is very small. the energy source at the galactic core is very small and can vary in brightness rapidly Seyfert galaxies are more likely to occur in pairs. gravitational interaction between galaxies distorts stellar motions feed matter into the central black holes Fig. 15-10 The supermassive black hole theory explains the energy sources in active galactic centers. Materials are being accreted into the central black hole. As matter spirals into the hole, it heats up, producing large amount of energy. High speed beams of gas are ejected perpendicular to the disk, forming the jets and lobes seen in many active objects. e.g., the dark disk with a central bright spot observed in the core of the galaxy NGC 4261 may be a disk of matter swirling into a central black hole (Fig. 15-9b).