1963

Quasars

Maarten Schmidt (b. 1929)

Like astronomers working in visible-light wavelengths (so-called optical astronomers), early radio astronomers were keen to survey the skies using their newly developed radio telescopes to identify the most interesting natural sources of radio waves. In addition to strong radio sources with well-identified optical counterparts—like the center of the Milky Way galaxy, or the Crab Nebula supernova remnant of 1054—astronomers in the 1950s began discovering hundreds of strong radio sources with no corresponding optical counterparts. Many appeared very small in the sky, almost starlike, but clearly not stars. Astronomers began calling them quasi-stellar objects, or quasars for short.

In 1962, the brightest quasar known, called 3C 273, passed behind the path of the Moon’s limb on multiple occasions, allowing radio astronomers to determine its position with very high accuracy. The Dutch-American astronomer Maarten Schmidt used this high-precision location data to search for and identify the quasar optically using the 200-inch (5-meter) Hale Telescope at Mount Palomar, which had become the world’s largest reflecting telescope since coming online in 1948. Schmidt took spectra of 3C 273 and in 1963 discovered that it was indeed not starlike at all, but rather showed Doppler-shifted emission lines from hydrogen—which suggested that it was both a great distance from us (following Hubble’s Law) and full of extremely high-speed (about 16 percent the speed of light!) ionized gas.

Quasars have since been discovered to be the brightest objects in the observable universe. The quasar 3C 273 turns out to be one of the closest to us, but it is still 2.4 billion light-years away, telling us that quasars are ancient features that were more common in the early history of the universe. Astronomers now believe that quasars are the violent inner regions of the centers of ancient active galactic nuclei, where enormous amounts of gravitational energy are being released as matter falls into black holes at the centers of quasar host galaxies. Energy from a spiraling-in disk of material is sometimes released in intense jets of radiation perpendicular to the disk; the brightest quasars appear to be sources where these luminous energy jets are pointed almost directly toward us.

Space artist Don Dixon painted this dramatic representation of two spiral galaxies colliding, feeding material into their central black holes to power intense, jet-like emission of radiation—quasars—in the central active galactic nucleus region surrounding each black hole.