OBSERVERS CHALLENGE JULY 2022
by Glenn Chaple
NGC 6210 Planetary Nebula in Ursa Hercules (Magnitude 8.8; Size 20” X 13”)
A majority of the non-Messier deep sky objects featured in the Observer’s Challenge were discovered by the German-English astronomer William Herschel during surveys conducted in the latter part of the 18th century and early years of the 19th. One of Herschel’s more notable “misses” was this month’s Challenge, the bright planetary nebula NGC 6210 in Hercules. Perhaps its relatively small size (a mere 20 by 13 arc-seconds and almost stellar-looking when viewed with low magnification) was to blame. But Herschel was able to detect the non-stellar appearance of Uranus when he discovered the planet in 1781, and its disc is just 4 arc-seconds across. Whatever the reason, NGC 6210 remained undetected until stumbled upon by the German-born Russian astronomer Wilhelm Struve while searching for double stars in 1825.
NGC 6210, nick-named the “Turtle Nebula” for its appearance in astroimages and visually through large-aperture scopes, is situated south of the “Keystone” of Hercules at 2000.0 coordinates RA 16h44m29.5s and Dec +23o47’59.5”. It’s about 4 degrees northeast of the 3rd magnitude star beta (β) Herculis, a good starting point for star-hoppers working with a low-power eyepiece (refer to Finder Chart B). You’ll know you’ve hit the mark when you arrive at a thin triangle 18 arc-minutes long and comprised of two 7th magnitude stars and a slightly out-of-focus 9th magnitude object (NGC 6210).
Even the smallest of astronomical telescopes will pick up NGC 6210. I first saw it on the evening of May 27, 1978, using a 3-inch f/10 reflector. In my logbook, I wrote, “At 30X, this object is still nearly star-like. At 60X, it seems more diffuse, and at 120X is definitely nebulous.” I saw no indication of color.
Recently, I returned to NGC 6210 with a 10-inch f/5 reflector. Again, low power (this time, 40X) revealed little more than a near-stellar image. A switch to higher magnification (208X) brought out a slightly bluish hue, but there was no sign of the outer extensions that form the “Turtle’s” head and appendages. I also failed to pick out the 13th magnitude central star. Darker skies (mine had a limiting magnitude of 5) and/or more aperture would have done the trick.
After giving NGC 6210 its due respect, turn your gaze to the 7th magnitude triangle member that lies 18 arc-minutes south and slightly west. This is the tight double star Struve 2094 (Σ2094). Its magnitude 7.5 and 7.9 component stars are just 1.1 arc-seconds apart, so I recommend using a scope with minimum aperture of 4 inches and a magnifying power of at least 200X on an evening when the seeing conditions are as steady as possible. An 11.7-magnitude third component lies 25 arc-seconds northwest of the main pair.
NGC 6210 is about 6500 light years away. The bright central portion is roughly one-half light year in diameter, while the “Turtle” spans 1.6 light years.
NGC6210 Finder Charts
A
B
Chart created using the AAVSO’s Variable Star Plotter (VSP). The location of NGC 6210 is marked with a crosshair. Numbers are stellar magnitudes, decimals omitted. The 2.8 magnitude star is beta (β) Herculis. Stars plotted to 10th magnitude. North is up in this 4 X 4 degree field. The star just below and slightly right of NGC 6210 is Struve 2094.
NGC 6210 Images
Image by Mario Motta, MD (ATMoB) taken with H alpha and O3 filters, each about 1 hour, through 32 inch scope with an ASI 6200 camera.
Sketch by Glenn Chaple (ATMoB)