Key Ideas

Quasars or active galactic nuclei (AGN): A quasar looks like a star but has a huge redshift. According to the Hubble law, these redshifts show that quasars are typically billions of light-years away from Earth.

• To be seen at such large distances, quasars must be very luminous, typically about 1000 times brighter than an ordinary galaxy.
• The peak of quasar activity took place when the universe was just over 2 billion years old; the era of quasar activity has long since passed.

Supermassive black holes power quasars: At the center of a quasar is a black hole with millions or even billions of solar masses.

• Rapid fluctuations in the brightness of quasars indicate that the region that emits radiation is quite small; the only known way to produce so much energy in such a small space is with a black hole.
• The Eddington limit relates the mass of a black hole to the maximum luminosity it can emit from its surroundings; if more luminous, radiation pressure would push away accreting gas and shut off the black hole’s fuel source.

Accretion and jets around black holes: Hot gas accreting around the black hole is thought to give a quasar its intense luminosity. About 10% of AGN also have strong radio emission coming from jets, as well as lobes of radio emission as the jets collide with surrounding gas on scales much larger than the host galaxy.

• Models for a thin accretion disk describe heat and emission produced as portions of gas on different orbits rub past each other, although no specific model of accretion has been verified.
• As gases spiral in toward the supermassive black hole, some of the gas may be redirected to form two jets of high-speed particles that are aligned perpendicularly to the accretion disk. When a jet is directed toward us, we can observe apparent superluminal motion.

Unified model of AGN: A thick dusty doughnut-shaped torus surrounds a quasar that, when viewed edge-on, blocks a direct view of the luminous accreting gas.

• A quasar is only visible if our viewing angle allows us to see the region in the center of the torus.
• When viewing the torus somewhat edge-on, the most luminous part of a quasar is blocked. Some emission is still observed, and the object appears as an active galaxy with a different emission spectrum.
• While the viewing angle changes what we observe, the central engine—an accreting black hole—is fundamentally the same. This insight unifies a diverse group of AGN.