Seek out 3C 273, the brightest optical quasar, in the spring sky

By Ade Ashford

This star field in the constellation of Virgo is unremarkable, except for the arrowed object. Known as 3C 273, this was the first quasar to be identified and the optically-brightest at magnitude +12.9. It is the extremely luminous core of an active galaxy powered by a supermassive black hole and lies a staggering 2,443 million light-years distant, receding from us at 14.6 percent of the speed of light. Amazingly, you can see this quasar in a 15-cm (6-inch) aperture telescope. Image credit: Ade Ashford.
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).

The so-called Bowl of Virgo is a happy hunting ground for galaxy devotees, containing a dozen Messier and scores of NGC objects. Exceedingly remote yet relatively bright is quasar 3C 273 located 4.7 degrees northwest of the beautiful double star gamma (γ) Virginis, otherwise known as Porrima. FW Virginis (variable from magnitude +5.6 to +5.8) and 16 Virginis (magnitude +4.9 to +5.0) lie either side of 3C 273, the area shown in greater detail down the page. Click the graphic for a PDF version of the chart suitable for printing. AN graphic by Ade Ashford.
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 a 15-cm (6-inch) telescope under dark skies. 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):

This detailed narrow-field finder chart for 3C 273 shows an area of sky just five degrees wide centred on the quasar with field stars down to magnitude +13. The inner circle is one degree wide, or the field of view of a typical eyepiece at 45 to 50× magnification. Stars FW and 16 Virginis are common to the wide-field Virgo chart to give you a sense of scale. Celestial north is up and east is to the left. Click the graphic for a PDF version suitable for printing. AN graphic by Ade Ashford.
Since it lies so close to the celestial equator, 3C 273 is equally visible from both the Northern and Southern Hemisphere. The 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. Evening skies will also be free from moonlight from the end of April into early May.

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.