NGC 7293: the Helix Nebula
October 2009 :
NGC 7293, the Helix Nebula, is the nearest planetary nebula (distance ~ 450 LY) and largest in apparent size (12 by 16 arcminutes). Moreover, it’s a 7th magnitude object. An easy telescopic target? Hardly! The magnitudes listed for deep-sky objects are often misleading, and the Helix Nebula is a prime example. Were you to defocus a 7th magnitude star until the image covers half a moon diameter, you’d have an idea of the visual appearance of the Helix. In Visual Astronomy of the Deep Sky, author Roger N. Clark notes that its average surface brightness is 20.8 magnitudes per square arcsecond.
Despite its faintness, the Helix Nebula can be readily observed. On a clear, moonless night in dark-sky areas, it may be glimpsed with binoculars. In fact some keen-eyed observers in extremely remote locations have spotted the Helix with the unaided eye! The key to viewing the Helix by telescope is to use a telescope/eyepiece combination that can produce a field about one-half to a full degree across. Because of its southerly location, you’ll want to select a viewing site with an open southern horizon, free of any sky glow.
I first saw the Helix on July 31, 1981 from the clear skies of Stellafane. Through my 3-inch f/10 reflector at 30X, it appeared as a “large, tenuous glow.” Stellafane regular Peter Kandefer peeked into the eyepiece and confirmed my sighting. More recently, I had no trouble spotting the Helix with a 4-inch f/8 reflector. The key in both instances was to know exactly where to look. The accompanying finder chart pinpoints the Helix Nebula’s location in the southern part of Aquarius about 1½ degrees west of the 5th magnitude star upsilon Aquarii.
The Helix Nebula offers three challenges: 1. Capture it with binoculars or small telescope; 2. Discern its annular form with medium to large-sized telescopes; 3. Spot its 13th magnitude central star. Are you up to the challenge? On the next clear, moonless autumn night, try your luck with NGC 7293, the Helix Nebula.
Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.
When to Observe
Constellations
The Helix Nebula, Space Telescope Science Institute
