quasi-stellar radio source

Quasar is a contraction of quasi-stellar radio source, a phrase coined in 1964 by Chinese-American astronomer Hong-Yee Chiu to label a class of celestial objects that had been detected by radio telescopes but looked, in optical photographs, like ordinary faint stars. Quasi is a Latin adverb meaning 'as if, almost, seemingly' — from quam (as) and si (if). Stellar derives from Latin stella (star). The compound quasi-stellar meant 'star-like but not quite a star': the objects appeared point-like in photographs, as stars do, but their radio emissions were inexplicably intense, and their optical spectra showed redshifts so extreme that they implied distances of billions of light-years. At those distances, to be visible at all, they had to be unimaginably bright. The abbreviated name — quasar — preserved the uncertainty of the coinage in a form short enough to use.

The story of quasars begins with radio astronomy, which emerged after World War II when surplus radar technology was repurposed for celestial observation. In the late 1950s, radio surveys of the sky catalogued thousands of discrete radio sources. Some could be matched to known objects — supernova remnants, radio galaxies. Others coincided with what appeared, through optical telescopes, to be blue-white stars. In 1963, astronomer Maarten Schmidt at Caltech measured the optical spectrum of one such source, 3C 273, and recognized with astonishment that its emission lines — the fingerprints of specific elements — were not merely shifted slightly from their expected positions but were redshifted by 15.8 percent. By Hubble's law, this implied a recession velocity of over 44,000 kilometers per second. The object was roughly two billion light-years away.

At two billion light-years, 3C 273 would have to be emitting energy equivalent to roughly a hundred times the entire Milky Way galaxy to appear as bright as it does. The power source was entirely mysterious in 1963. The now-accepted explanation — a supermassive black hole accreting infalling matter at the center of a distant galaxy — was not seriously proposed until the mid-1960s and was not confirmed as the dominant mechanism until the 1990s. As matter spirals into a black hole's accretion disk, gravitational energy converts to heat and radiation with extraordinary efficiency: up to 40 percent of the infalling mass can be converted to energy, compared with less than one percent in nuclear fusion. The black hole at 3C 273's center is estimated to contain about six billion solar masses and was once accreting at a rate that would produce the observed luminosity.

Quasars are almost exclusively distant objects — they were far more common in the early universe, when galaxies were younger and their central black holes were still actively feeding. The nearest quasar, 3C 273, is still roughly 2.4 billion light-years away; most are billions of light-years farther. This means that observing quasars is also a form of time travel: we see them as they were billions of years ago. Modern surveys have catalogued over a million quasars. Their redshifts have made them indispensable probes of cosmological structure — the light they emit passes through intergalactic gas clouds on its way to us, and the absorption patterns tell astronomers about the composition and distribution of matter across cosmic time. The quasi-star, with its improvised name, turned out to be one of the universe's most informative instruments.