Gemini’s Nebula Twins

Near Castor and Pollux is a strange double nebula.

Planetary Nebula NGC 2371

This Hubble Space Telescope image provides a close-up view of planetary nebula NGC2371-72. NASA/STScI/AURA

With Gemini high in the sky during winter months, an odd double nebula is in prime viewing location: NGC2371 and NGC2372. But is this two deep-sky objects or just one? It can certainly look like two faint fuzzies close together. But NGC2371-72 is a single bipolar planetary nebula that features twin lobes of ejected gas on either side of a decaying star. I call it the Peanut Planetary.

Not many bipolar planetaries flaunt their form in a small telescope. The best, by far, is the Dumbbell Nebula (M27), in Vulpecula. M27 is an hourglass-shaped cloud of magnitude 7.4 and nearly 50 square arc minutes in area. Second prize goes to the Little Dumbbell (M76), in Perseus, but it’s magnitude 10.1 and only 1.7 arc minutes long. The one in Gemini is a “mini-mini dumbbell” that glows wanly at magnitude 11.2 and spans less than 1 arc minute of sky.

The great English observer William Herschel discovered the Peanut in 1785 using an 18¾-inch reflector. His log entry for H3162 reads, in part: “Two, faint of an equal size, both small, within a minute of each other [whose] apparent atmospheres run into each other.” A century later, Herschel’s dim duo became numbers 2371 and 2372 in J. L. E. Dreyer’s New General Catalogue. Nowadays, the twin fuzz in Gemini is accessible to the average backyard telescope—in my case, a 10-inch f/5.5 Dobsonian. The trick is locating it.

NGC2371-72 finder chart

Gemini is a prominent constel­lation in winter and spring skies. Below its two luminaries, Castor and Pollux, crouches an unusual planetary nebula the author has dubbed the Peanut because of its “pinched 8” shape. (Click on the chart for a full-size version.)  Main chart cartography by Glenn LeDrew

My orientation begins with 1.2-magnitude Pollux and 1.6-magnitude Castor. Blue white Castor, a showpiece binary boasting 1.9- and 3.0-magnitude components 4.2 arc seconds apart, is striking in my 10-inch at 82×. Pollux is a single, dusty yellow sun, and it’s my star-hop starting point. From Pollux, I aim southwest about 2 degrees to orangey red upsilon (υ), then turn northwest for 2½ degrees to golden iota (ι)—both stars are magnitude 4. Employing 58×, I centre iota in the scope and push northward for 1⅔ degrees in search of Herschel’s wee wisp. If that doesn’t work, I retreat to the fifth-magnitude stars 64 and 65 Geminorum, almost 1 degree east of iota. Set ¼ degree apart, 64 and 65 make good pointers, because they aim almost directly at the target nebula six “lengths” away.

During my observation the nebula was in the east, where my suburban sky is darkest. NGC2371-72 materialized at 58× as a minute rectangular patch. At 116×, I confirmed that the rectangle is slanted northeast southwest and pinched in the middle. Interestingly, the southwest end looked brighter, and perhaps bigger, than its counterpart. At 200×, the object was diffuse and so strongly pinched, it seemed to be two lobes in contact—hence my dubbing it the Peanut. But the peanut shell broke in half when my patient averted vision resolved the planetary into two distinct parts. In other words, I detected a very narrow space separating the lobes. In reality, the lobes don’t overlap; they’re connected by a thin bridge of nebulosity that I couldn’t quite see (but is obvious in the Hubble Space Telescope image, above). Not surprisingly, the planetary’s 15th-magnitude central star eluded my gaze completely.

NGC 2371-72 Karlsson

Not one of the brighter targets for backyard telescopes, NGC2371-72 is just above centre in this eyepiece impression by observer John Karlsson. It illustrates why a famous astronomer once said, “Amateur astronomers are connoisseurs of smudges.” Sketch by John Karlsson

I repeated the above steps with a doubly ionized oxygen (O-III) filter. What a difference! Even at 58×, the elongated patch appeared sharply defined and bright at one end. At 116×, the inequality of the lobes was obvious and the space between them was very dark. At 200×, the prominent lobe contained a starlike knot. An Ultra High Contrast (UHC) filter reproduced these details almost as effectively, and the lighter field of view was easier on my eye. Also, the lobes seemed naturally diffuse and extended, which, in contrast to the O-III view, permitted their “apparent atmospheres [to] run into each other.” So the Peanut Planetary lives after all. It certainly does in the sketch at left, made by eagle-eyed observer John Karlsson, who used an 8-inch Dob at 120× with an O-III filter.

Years ago, I scrutinized NGC2371-72 from a wilderness site using my 17.5-inch Dobsonian at 300×, no filters. I remember seeing a stubby arc of nebulosity extending from each end of the binary blob. I glimpsed the central star too. Yet I wasn’t able to fully trace the bridge of tenuous haze that links the lobes. Even with a big scope under a black sky, the Peanut Planetary is one tough nut to crack!

SkyNews associate editor Ken Hewitt-White has observed deep-sky fuzzies over southern British Columbia for more than four decades.

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