One Good Target With Some Other Sights Worth Seeing While You’re in the Neighborhood
September The Blinking Planetary (NGC 6826) with side trips to a nicely-balanced pair of Sun-like stars, a colorful optical triple for binoculars, and a deep red carbon star
Flying across the zenith early on September evenings, the constellation Cygnus (The Swan) is home to dozens of wonderful and amazing celestial sights. One of them is The Blinking Planetary (NGC 6826), mag 8.8 and about 30” across, conveniently located along The Swan’s western wing. To get there, start at mag 3.8 Iota () Cygni, second-to-last star on the western wing – don’t confuse it with Kappa () Cygni, also mag 3.8, which marks the tip of the wing. From Iota, move your finderscope to mag 4.5 Theta () Cygni, easily identified as the brightest point of a nearly-equilateral triangle. (You may see a slightly red star just 5’ from Theta: that’s R Cygni, an S spectrum variable whose brightness ranges from mag 6.1 to 14.4). Centering Theta will bring into view 16 Cygni, whose brightness is almost identical to the other two stars of Theta’s triangle. If you take a minute to examine 16 Cygni closely, you’ll find that it’s an easily-split pair of Sunlike yellow dwarf stars: 6.0 + 6.2 sep 39”.
This month’s targets (stars to mag 5.5) North is up
The nebula is just a half degree due east of 16 Cygni – but in your finderscope or at low power, it will look like just another star, so you’ll have to use the chart below to pick it out. Once you’ve identified which “star” is really the nebula, center it in the finder field, switch to the eyepiece view, and bump the power up to at least 100x to make the “star” blossom into a nebulous disk using averted vision. Depending on the size of your scope, you may also detect hints of the nebula’s blue-green color. And now for the magic: to make the nebula “blink,” look at it with direct vision, then look to the side and catch it with averted (peripheral) vision. With direct vision, the nebula disappears, leaving only the mag 10.4 central star; with averted vision, the nebula pops into view, because your eye’s rods – which are used in averted vision – are more sensitive to low light than the cones, which are used in direct vision. Shifting back and forth between direct and averted vision makes the nebula “blink” on and off. This effect occurs in scopes as small as 3”, but becomes less pronounced in 10” or larger instruments. With large scopes at high power, look instead for bright knots east and west of the central star.
5° fields with 1° insets Stars to mag 8.0 (mag 8.8 in 1° fields) North at top Selected magnitudes shown – decimals omitted
What happens with The Blinking Planetary is entirely different from the “blink technique” that is sometimes used when an observer is having trouble identifying which object in the field is a planetary nebula. In “blink technique,” you hold an O-III filter between thumb and forefinger, and move it in and out of the light path between eye and eyepiece. The filter “lights up” the planetary nebula (due to its oxygen content) while suppressing everything else, making the nebula stand out.
While you’re in the neighborhood, look about 2/3 of the way from Iota Cygni to Deneb (Alpha Cygni), where you’ll find two naked-eye stars, one above the other. The one farther south, Omicron-1 (1) Cygni, also known as 30-31 Cygni, is a wide and colorful optical triple: 31 Cygni consists of a mag 3.8 orange primary 107” away from a blue mag 6.7 secondary, with a blue-green mag 4.8 star (30 Cygni) 338” away. The three stars are unrelated, but lie on adjacent lines of sight from our perspective. Splitting 30 from 31 is a challenge for naked- eye observers. The trio can be split in most binoculars, and split easily at low power in any telescope.
Before heading home, point your finderscope midway between Omicron-1 Cygni and mag 2.9 Delta () Cygni, marking the west arm of The Northern Cross, where you’ll find a prominent mag 6 star. Center the mag 6 star and look 1½° south to find a remarkably red star, AX Cygni, a red giant carbon star whose color is enhanced by carbon in its atmosphere, which scatters blue light but lets the red light pass through. The star’s magnitude varies from 7.4 to 8.8 in an irregular cycle, so it might not always be easy to see in your finder – but its intense color is worth the effort to hunt it down.
Rick Gering / September 2020