273 (two hundred [and] seventy-three, CCLXXIII) is the natural number following 272 and preceding 274.

It is a sphenic number, and a repdigit in base 9 (333), base 16 (111), base 20 (DD or 13 13), base 38 (77), and base 90 (33).

273 is also:

* The zero of the Celsius temperature scale is (to the nearest whole number) 273 Kelvin. This corresponds with absolute zero (0 K) being approximately -273°C.
* American composer John Cage's piano composition 4′33″ consists of 273 seconds (four minutes and 33 seconds) of silence, in three movements.
* The year AD 273 and 273 BC.
* The death toll of the air crash of American Airlines Flight 191 is 273 people (271 on board,2 on ground).

3C 273, the object number 273 in the third Cambridge Catalogue of radio sources, is the brightest and at least one of nearest of all quasars (acronym for "QUASi-stellAR radio sources"), i.e. it appears stellar in telescopes. This one shows its spectral lines shifted to the red by 16 %, so that this object appears to recede from us at 16 % of the speed of light, or about 48,000 km/sec. Applying Hubble's expansion law, this recession velocity corresponds to a distance of about 640 Mpc, or about 2 billion light years (H0=75 assumed; H0=60 would correspond to a distance of 2.6 billion light years).

At this enormous distance, light fades by 38.9 magnitudes (for H0=75, or 39.5 mag for H0=60), so its average apparent magnitude of 12.8 corresponds to an enormous absolute brightness of -26.1 (-26.7) magnitudes visually. So from a distance of 10 parsecs, this object would shine in the sky about as bright as our sun ! This quasar's luminosity is, therefore, about 2 trillion (10^12) times that of our sun, and still about 100 times that of the total light of average giant galaxies like our Milky Way !

For most amateurs, this is the most remote object they can view with their (or their friends') telescopes. It sometimes brightens to brighter than 12th magnitude (John Isles reports peaks of up to 11.7 mag), but can fade to 13.2. All other quasars are normally below 14th magnitude, but some can have brighter outbursts (Isles lists the following other quasars with peaks brighter magnitude 13: PKS 0537-441 [12.1-17.0], 3C 279 [11.0-17.7], and PKS 1510-089 [1.6-17.6], while in February 1998, it was reported that the nucleus of Markarian 421 had an outburst to about 12th magnitude).

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