As any keen Northern Hemisphere deep-sky observer knows, the arrival of spring means that it’s open season for galaxies. This is the time of year when you can while away moonless nights trawling the constellations of Leo, Virgo and Coma Berenices, where an 8-inch (20-cm) aperture telescope (or larger) will net you literally hundreds of catalogued galactic goodies.
While the distances to the centres of these galactic conglomerations are truly mind-boggling – 54 million and 320 million light-years, respectively, for the Virgo and Coma Clusters – they are relatively close compared to a peculiar object designated 3C 273, found within the so-called Bowl of Virgo bounded by epsilon, delta, gamma, eta and beta Virginis, plus beta Leonis (Denebola).Named for its ranking as the 273rd object in the Third Cambridge Catalogue of Radio Sources published in 1959, occultation studies of 3C 273 reveal it to be associated with an unresolved stellar object having an apparent magnitude of +12.9. A redshift of 0.158 reveals that 3C 273 is much farther than either the Virgo or Coma Clusters – a staggering 2,443 million light-years away.
To have a magnitude of +12.9 at such a distance implies that the quasar has a phenomenal luminosity – so bright, in fact, that 3C 273 is at least 4,000,000,000,000 times more luminous than the Sun at visible wavelengths. We now know that such a quasar is an extremely luminous active galactic nucleus (AGN), powered by a supermassive black hole. And despite all subsequent quasar discoveries, 3C 273 remains the brightest quasi-stellar object at optical wavelengths.
Finding and identifying 3C 273
Given that this quasar is slightly brighter than magnitude +13, it’s a viable target for 15-cm (6-inch) telescopes and larger. The easiest way to locate 3C 273 is with a computerised GoTo telescope, or an instrument equipped with digital setting circles. Its J2000 coordinates are α=12h 29.1m, δ=+02° 03′ (α=12h 30.1m, δ=+01° 57′, J2019.2). However, even if your computerised mount automatically takes you to the correct location 3C 273 is indistinguishable from faint foreground stars, so you still need a detailed finder chart like the one below (click here for a PDF version):Since it lies so close to the celestial equator, 3C 273 is equally visible from both the Northern and Southern Hemisphere. This quasar currently transits 38° above the southern horizon in the small hours of the morning for an observer in the heart of the British Isles, but by the end of March it transits close to midnight. Note that full Moon occurs on 21 March, so moonlight will be a problem from mid-month.
Once you do home in on the correct spot to see this speck of light, take time to contemplate that 3C 273 is the extremely luminous core of an active galaxy powered by a supermassive black hole. It’s receding from us at 14.6 percent of the speed of light and lies 45 times farther away than the heart of the Virgo Cluster of galaxies. It may well be that 3C 273 is the most distant celestial object you’re ever likely to see with a modest aperture telescope.