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November Object of Month

John Hobbs  | Published on 11/24/2021

OBSERVER’S CHALLENGE* – NOVEMBER, 2021

by Glenn Chaple

NGC 7662 – Planetary Nebula in Andromeda (Magnitude 8.3, Size 37”)

            Last month’s Observer’s Challenge focused on NGC 6857, an emission nebula that astronomers once mistook for a planetary nebula. Our November Observer’s Challenge, NGC 7662 in Andromeda, is a bona fide planetary nebula. It was discovered by William Herschel on October 6, 1784, one month after he found NGC 6857. At a magnitude of 8.3, NGC 7662 is a full 3 magnitudes brighter than NGC 6857. It’s one of the brightest of all deep sky objects in its class, easily seen in a small scope. So what is its challenge?

If you’re a novice backyard astronomer, even the brightest and easiest planetary nebula can test your developing observing skills. These objects are small and will appear stellar at low magnifications. Begin your NGC 7662 quest at “Frederick’s Glory,” a Y-shaped asterism in the northwest part of Andromeda (refer to Finder Chart A). Using a low-power eyepiece and Finder Chart B, start at iota (ι) Andromeda, the 4.3-magnitude star on the chart. From there, move 2 degrees westward until the 6th magnitude star 13 Andromedae (the unlabeled star one-half degree northeast of NGC 7662) enters the field.  Switch to a medium-power eyepiece (60X works fine) and sweep the area around 13 Andromeda until NGC 7662 comes into view as a small out-of-focus star. Center it in the field of view and switch to the highest magnification your telescope aperture and seeing conditions allow. Owners of GoTo scopes can “cheat” by punching in the celestial coordinates Right ascension 23h 25m 54s, Declination 42° 32’ 6” and slewing straight to the target.

Here’s a fact about NGC 7662 that I haven’t mentioned. It’s noted for its blue color, hence the popular nick-name, the “Blue Snowball.” I was unable to detect any color at all when viewing NGC 7662 with a 60mm (2.4-inch) refractor, but the color was vivid when I viewed it with an 18-inch Dob. What is the smallest aperture that will bring the “Blue Snowball” to light? For that matter, what is the smallest aperture that reveals its 13th magnitude central star?

Challenge yourself by looking for NGC 7662 with binoculars. Using Finder Chart B as a guide, you should come across an 8th magnitude “star” in the position indicated on the chart. Reasonably dark skies will be a must if you’re working with standard 7X30s or (better yet) 7X50s.

            As is the case with many planetary nebulae, the distance to NGC 7662 is uncertain at best. Calculations fall between 1800 and 5600 light light years. I’ll settle on a figure of 2500 light years, given by NASA and the Universe Guide website (universeguide.com). The latter source includes an interesting table that shows the time needed to arrive at NGC 7662 by various means of travel. Light speed gets you there in 2500 years. The New Horizons Probe, which took 15 years to reach Pluto would require 51 million years. A Mach 2 jet airliner would reach its destination in a little over 1 billion years, while a speeding (120 mph) car would require nearly 14 billion years, not counting a lot of stops for gas!. Want to take a stroll to the Blue Snowball? If you leg it out at a 15-minute-per-mile pace, plan on around 420 billion years! I don’t know about you, but I’m sticking to my backyard and a telescope.

Image by Mario Motta