Which quasar was the first to have its redshift identified, revealing its great distance? The answer is 3C 273.
Located in Virgo, 3C 273 became the first object recognized as a quasar after Maarten Schmidt and Bev Oke reported its unusually large redshift in 1963. Its redshift, about 0.158, showed that the seemingly star-like radio source was billions of light-years away rather than a nearby star.
The source had first been precisely located through lunar-occultation observations at the Parkes Radio Telescope. Its optical counterpart looked stellar, but its spectrum contained emission lines that initially resisted identification. The redshift solved the mystery and established quasars as extraordinarily luminous distant active galactic nuclei.
A common mix-up is 3C 48: it was associated with an optical counterpart earlier, but its redshift was measured after 3C 273’s breakthrough. 3C 273 is also the optically brightest quasar in Earth’s sky and has a prominent relativistic jet.