Astronomy under city lights – Alan Plummer reports
Night time observing at Sydney Observatory, image Nick Lomb
Some astronomers are not dissuaded by light pollution. Observers of the Sun and the Moon, and the night-sky guides at Sydney Observatory are important contributors to astronomy. Variable star observers also can work from under city lights or bright moonlight. In order to demonstrate this, your pioneering writer left his dark sky at home for a session of variable star observing from Sydney Observatory, almost in the heart of the CBD. Very successful it was, too, and great fun. Here’s the story.
I began at 9pm on a Monday after a meeting of the Sydney City SkyWatchers, with the Observatory’s guide-lecturer [and frequent contributor to this blog - Nick] Les Dalrymple already under way with with the public viewing night. The sky looked better than I had hoped; the constellations of Orion, Canis Major, Puppis, Vela and the Southern Cross all visible, as were the Harbour Bridge, Luna Park and the CBD. Les later told me that the sky was ‘about average’.
After setting up a 30cm Newtonian telescope plus 50mm and 70mm binoculars along side Les and the visitors, the first target on my list was T Pyxidis. This is a recurrent nova that undergoes a thermonuclear runaway explosion every 19 years or so, but is more than 20 years overdue today. I had not expected to find this field in the sky-glow because the pointer stars I use were not visible to the naked eye. However, the telescopes’ finder was up to the task, and an observation of <12.5mv (less than 12.5 visual magnitude) was logged. If in outburst, T Pyx would have been clearly visible, even well before maximum, and the worldwide astronomical community would have been notified at once.
Three of the brighter dwarf novae were observed, although not in outburst; Z Chamaeleontis at <12.8mv, V1159 Orionis at <13.5mv and HL Canis Majoris at <13mv. These negative observations are useful, because they can define the starting point of the next outburst. Finding V1159 Ori and HL CMa is a pleasure, with the famous Orion nebula as a finder object for the first, and the star Sirius actually in the eyepiece field for HL CMa. To be honest, that’s a nuisance, but Sirius is a beautiful object anyway. And incidentally, the Orion nebula was visible to the naked eye, and was pointed out to the public.
A series of Cepheid variable stars were observed from 3rd to 10th magnitude with the binoculars and the telescope. These are pulsating supergiant stars that populate the Galactic plane, which was overhead at the time. Though rare, they are very luminous and so many are accessible to amateurs.
The famous Luminous Blue Variable (LBV) Eta Carinae was observed, both by the public and me, along with other LBVs AG Car, V432 Car and S Doradus. These are not super but hypergiants, and are so rare that all of them are well known by name to variable star observers. Eta Car and AG Car were binocular objects under city lights from far across our Galaxy, and S Dor is so luminous that it’s a binocular object from another galaxy, the Large Magellanic Cloud (it was 9.1mv on the night).
V432 Car warrants a special mention. The observation, 11.5mv through the telescope, was sent immediately to the University of Brussels in Belgium, to contribute to a professional study of the object involving the European Southern Observatory in Chile, and this observation will be part of the published work. In fact, all of the observations taken here are valuable and are made available to the astronomical community.
In total, 18 observations made over 50 minutes, taking place while Les was showing the wonders of the sky to the Observatory visitors. To hear the constant exclamations of wonder at the objects and explanations of their nature was heartening. Plus an unexpected treat; a very bright orange shooting star was seen underlining the Southern Cross and disappearing behind the CBD.
So the point is made: If you are interested in astronomy and live in the city, don’t be discouraged, even though to be honest there is a lot you really can’t do. However, given that in your backyard you might not be doing the public education work that Sydney Observatory does, maybe valuable scientific research is the only thing that you can do!
Alan Plummer (variable star observer and member of the Sydney City Skywatchers)
Notes
1. For reasons of convenience and consistency I had my own instruments: a 30cm F5 Newtonian telescope with 32mm, 21mm, 9mm and 5mm wide field eyepieces, 50mm and 70mm binoculars, plus my charts and associated things.
2. More complete descriptions of all the objects and observations. There was Z CMa (a FU Ori – T Tau type binary system), in the bright state at 8.9mv. Also observed was S Car, a bright Mira at 5.6mv.
