Summary of Key Ideas

• The development of radio astronomy in the late 1940s led to the discovery of very powerful and extremely distant energy sources.

Quasars and Other Active Galaxies

• An active galaxy is an extremely luminous galaxy that has one or more unusual features: an unusually bright, starlike nucleus; strong emission lines in its spectrum; rapid variations in luminosity; and jets or beams of radiation that emanate from its core. Active galaxies include quasars, Seyfert galaxies, radio galaxies, double-radio sources, and BL Lacertae objects.
• A quasar, or quasi-stellar radio source, is an object that looks like a star but has a huge redshift. This redshift corresponds to a distance of billions of light-years from Earth, according to the Hubble law.
• To be seen from Earth, a quasar must be very luminous, typically about 100 times brighter than an ordinary galaxy. Relatively rapid fluctuations in the brightness levels of some quasars indicate that they cannot be much larger than the diameter of our solar system.
• An active spiral galaxy with a bright, starlike nucleus and strong emission lines in its spectrum is categorized as a Seyfert galaxy.
• An active elliptical galaxy is called a radio galaxy. It has a bright nucleus and a pair of radio-bright jets that stream out in opposite directions.
• BL Lacertae (BL Lac) objects (some of which are called blazars) have bright nuclei whose cores show relatively rapid variations in luminosity.
• Double-radio sources contain active galactic nuclei located between two characteristic radio lobes. A head-tail radio source shows evidence of jets of high-speed particles that emerge from an active galaxy.

Supermassive Central Engines

• Many galaxies contain huge concentrations of matter at their centers.
• Some matter that spirals in toward a supermassive black hole is squeezed into two oppositely directed beams that carry particles and energy into intergalactic space.
• The energy sources from quasars, Seyfert galaxies, BL Lac objects, radio galaxies, and double-radio sources are probably matter ejected from the accretion disks that surround supermassive black holes at the centers of galaxies.