3C2733C273 is a quasar located in the constellation Virgo. It is the optically-brightest quasar in our sky (m ~ 12.9), and one of the closest with a redshift, z, of 0.16. The name signifies that it was the 273rd object (ordered by right ascension) of the Third Cambridge Catalog of Radio Sources (3C), published in 1959. The radio source was quickly associated with an optical counterpart, an unresolved stellar object. In 1963, Maarten Schmidt and Bev Oke published a pair of papers in Nature reporting that 3C273 has a substantial redshift, placing it several billion light years away.
Prior to the discovery of 3C273, several other radio sources had been associated with optical counterparts, the first being 3C48. Also, many active galaxies had been misidentified as variable stars, including the famous BL Lac, W Com, and AU CVn. However, it wasn't understood what these objects were, since their spectra were unlike those of any known stars. 3C273 was the first object to be identified as what we now know quasars to be -- extremely luminous objects at cosmological distances.
3C273 is a radio-loud quasar, and was also one of the first extragalactic X-ray sources discovered in 1970. The luminosity is variable at nearly every wavelength from radio waves to Gamma rays on timescales of a few days to decades. Polarization has been observed in radio, infrared, and optical light, suggesting that a fraction of the emitted light is synchrotron radiation, created by a jet of charged particles moving at relativistic speeds. Such jets are believed to be created by the interaction of the central black hole and the accretion disk. VLBI radio observations of 3C273 have revealed proper motion of some of the radio emitting regions, further suggesting the presence of relativistic jets of material.
3C273 is located at (J2000) right ascension 12h 29m 6.7s, declination +2d 3m 8.6s, and is visible in May in both the northern and southern hemispheres. It is bright enough to be observed with larger amateur telescopes.