quasar

In the early 1960s, astronomers found objects that looked like faint blue stars but had spectra no star could make, and were beaming out radio waves. They were dubbed 'quasi-stellar radio sources' — star-like, but not stars — soon shortened to quasars. The puzzle was solved when their spectral lines turned out to be hugely redshifted: these 'stars' were actually the blazing cores of galaxies billions of light-years away.

A quasar is simply the most luminous kind of active galactic nucleus: a supermassive black hole feeding so fast that its accretion disk outshines the entire surrounding galaxy by a factor of a hundred or more, often reaching 10^39 to 10^41 watts. Because they are so bright, we can see them across almost the whole observable universe, which means we see them as they were billions of years ago. Their huge redshifts — light stretched by the expansion of space on its long journey to us — place the most distant known quasars in an era when the universe was under a billion years old. The point-like, star-like look is just because the brilliant nucleus drowns out the much fainter host galaxy.

Quasars are cosmic lighthouses for studying the early universe. Their light, crossing billions of light-years, passes through intervening gas clouds that imprint absorption lines, letting us map matter we could never see directly (for example the Lyman-alpha forest). They also tell us that supermassive black holes were already growing vigorously when the cosmos was young. Note that 'quasar' is not a separate type of object from an AGN — every quasar is an AGN; 'quasar' just names the brightest, usually most distant, examples. Not all quasars are even strong radio sources, despite the original name.