Novas are popping up like daisies in the constellation Sagittarius this year. To date, four “new stars” have been discovered in the Teapot, enough to keep its contents at a steady boil.
Earlier yesterday (July 7) Koichi Nishiyama and Fujio Kabashima of Japan found the most recent nova – Nova Sagittarii 2012 #4 – on exposures made with a camera and 105 mm lens. Shining at magnitude 7.7 at the time of discovery, the nova was bright enough to see in binoculars and small telescopes. It still is. Last night I saw it from my driveway at the identical brightness through my 8x40s.
Typical 40mm and larger binoculars can reach 8th magnitude or fainter; with a good locator map, you should be able to see the nova for yourself in the coming nights, especially since the moon is rapidly departing the evening sky.
As we learned a few days ago in the case of Nova Persei, novas occur in close binary stars, where a small but extremely dense and massive (for its size) white dwarf grabs hydrogen gas from its closely orbiting companion. The gas swirls down to the dwarf’s 150,000 degree F surface, where it’s compacted by the gravity and heated until detonating like a gazillion thermonuclear bombs. Suddenly, a faint star that wasn’t on anyone’s radar vaults a dozen magnitudes to become a standout “new star”, one bright enough for a pair of Japanese amateurs to snag it in a 40-second exposure.
Novae can rise in brightness from 7 to 16 magnitudes, the equivalent of 50,000 to 100,000 times brighter than the sun, in just a few days. Meanwhile the gas they expel in the blast travels away from the binary at up to 2,000 miles per second. This is one big firecracker!
Depending on the particulars of the explosion, including distance and whether or not there’s a significant amount cosmic dust between us and the star, novae can be bright or faint.
Some, like Nova Cygni 1975 (in the Northern Cross) reached magnitude 1.7 in late August that year. I remember how its presence “distorted” the outline of that familiar constellation. Other novae are too faint for small instruments or brighten and fade so quickly, if there’s a cloudy spell, you might miss it.
Sagittarius – and nearby Scorpius and Ophiuchus also – are familiar discovery grounds for novae because that’s where most of the stars of the Milky Way are concentrated. Why? Because we’re facing the center of the galaxy when we direct our gaze toward these star groups. The more stars in your line of sight, the better the chance that one of the many billions will flare into nova-hood.
The best time to see Nova Sgr 2012 #4 is between about 10:30 p.m. and 3 a.m. Optimum time is around midnight when Sagittarius, always low in the southern sky from northern latitudes, reaches its highest point above the horizon. Use the maps to star-step your way to the nova. For those familiar with celestial coordinates, its exact position is: R.A. 18 h 20′ 27.3″, Dec. -27 degrees 44′ 26″.
You can also get a nice printable chart at the American Association of Variable Star Observers (AAVSO) website. In the Star Finder box, type in PNV J18202726-2744263, the temporary name for the new nova, and click the Create a finder chart link. Your chart will have north at top. If you want to print out a different orientation or a more customized version, just click the blue Return and Replot link.
How bright the nova will be tonight is anybody’s guess. Maybe the same as last night, but it could also be fainter or brighter. Good luck spotting one of nature’s more explosive creatures!