3C 273 is a quasar located at the center of a giant elliptical galaxy in the constellation of Virgo. It was the first quasar ever to be identified and is the visually brightest quasar in the sky as seen from the Earth, with an apparent visual magnitude of 12.9, outshining by more than 16 times the entire galaxy that hosts it. The derived distance to this object is 749 megaparsecs (2.4 billion light-years). The mass of its central supermassive black hole is approximately 900 million times the mass of the Sun.

3C 273 is visible from March to July in both the northern and southern hemispheres. Situated in the Virgo constellation, it is bright enough to be observed by eye with a 6-inch (150 mm) amateur telescope. Due in part to its radio luminosity and its discovery as the first identified quasar, 3C 273's right ascension in the Fifth Fundamental Catalog (FK5) is used to standardize the positions of 23 extragalactic radio sources used to define the International Celestial Reference System (ICRS).

Given its distance from Earth and visual magnitude, 3C 273 is the most distant celestial object average amateur astronomers are likely to see through their telescopes.

This is the optically brightest quasar in the sky from Earth with an apparent visual magnitude of ~12.9, and one of the closest with a redshift, z, of 0.158. A luminosity distance of D_L = 749 megaparsecs (2.4 billion light-years) may be calculated from z. Using parallax methods with the Very Large Telescope interferometer yields an angular distance estimate of D_A=1.80+0.32
−0.28 Gly (552+97
−79 Mpc). The latter distance is related to the comoving distance as ${\displaystyle D_{L}=(1+z)^{2}D_{A}}$.

It is one of the most luminous quasars known, with an absolute magnitude of −26.7, meaning that if it were only as distant as Pollux (~10 parsecs) it would appear nearly as bright in the sky as the Sun. Since the Sun's absolute magnitude is 4.83, it means that the quasar is over 4 trillion times more luminous than the Sun at visible wavelengths.

The luminosity of 3C 273 is variable at nearly every wavelength from radio waves to gamma rays on timescales of a few days to decades. Polarization with coincident orientation has been observed with radio, infrared, and optical light being emitted from a large-scale jet; these emissions are therefore almost certainly synchrotron in nature. The radiation is created by a jet of charged particles moving at relativistic speeds. VLBI radio observations of 3C 273 have revealed proper motion of some of the radio emitting regions, further suggesting the presence of relativistic jets of material.

This is a prototype of an Active Galactic Nucleus, demonstrating that the energy is being produced through accretion by a supermassive black hole (SMBH). No other astrophysical source can produce the observed energy. The mass of its central SMBH has been measured to be 886±187 million solar masses through broad emission-line reverberation mapping.

The quasar has a large-scale visible jet, which measures ~200,000 light-years (61 kpc) long, having an apparent size of 23″. Such jets are believed to be created by the interaction of the central black hole and the accretion disk. In 1995, optical imaging of the jet using the Hubble Space Telescope revealed a structured morphology evidenced by repeated bright knots interlaced by areas of weak emission. The viewing angle of the jet is about 6° as seen from Earth. The jet was observed to abruptly change direction by an intrinsic angle of 2° in 2003, which is larger than the jet's intrinsic opening angle of 1.1°. An expanding cocoon of heated gas is being generated by the jet, which may be impacting an inclined disk of gas within the central ~ 6 kpc.