DSO of the Month
3C 273
AKA: PGC 41121; HIP 60936
Position: 12 hr 29.1 min 02 deg 03 min 09 sec
Due south at 21.55 (BST) on 15 May 2021
3C 273 with a jet coming out of it.
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
At some point when you are outside looking at this or that deep sky object, the thought may have crossed your mind: can I see a quasar from my back garden? The answer, perhaps surprisingly, is yes – as long as you have a large enough telescope. The optically brightest quasar, 3C 273, has a magnitude of 12.9 (although Simbad lists V as being 14.8) and should be visible in a 200mm telescope and certainly in a 300mm telescope. It should also be possible to image it in a smaller telescope. It lies in northern Virgo (in the so-called bowl of Virgo) between the bright stars Spica and Denebola and forms a triangle with Porrima (Gamma Virginis) and Zaniah (Eta Virginis), but you will need to use go-to to locate it as it is so faint. Do not expect to see any detail; it will just be a blue star-like object. In the Hubble Space Telescope, however, it is clearly seen as a giant elliptical galaxy with a jet shooting away from it. While its appearance may seem disappointing, it is 2,443 million light years away from us (with a redshift of 0.16) and it is moving away from us at 15% of the speed of light. It is thus easily the most distant object that you are ever likely to see in your telescope! For more details for observing it with a finder chart, read https://astronomynow.com/2019/03/07/seek-out-3c-273-the-brightest-optical-quasar-in-the-spring-sky/.
While it may be very dim in visible light, it is a powerful emitter of radio waves and as such, it was radio source 273 in the third Cambridge Catalogue of radio sources published in 1959 (hence the designation 3C 273). Eleven years later, it was also shown to be one of the most powerful X-ray sources outside our own galaxy. For many years, its energy source (and even its distance) was a mystery – it was either a nearby object which was a significant source of radio energy but otherwise unremarkable or an extremely distant object producing an utterly gigantic amount of energy. We now know of course that this energy is generated by a supermassive black hole feeding on the material surrounding it, hence the jet coming out of the quasar (which is short for quasi-stellar object). The black hole lives inside a galaxy but such the distance involved, the galaxy itself was invisible until very large telescopes were available to observe the galaxy. Part of the problem was the sheer brightness (relatively speaking) of the galactic centre, which means a coronagraph has to be used to block out the core to enable the rest of the galaxy to be seen.
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