NGC 6572, Planetary Nebula in Ophiuchus: July 2021 Observer’s Challenge Report #150

Roger Ivester, North Carolina

&

Sue French, New York

July 2021

Report #150

NGC 6572, Planetary Nebula in Ophiuchus

Sharing Observations and Bringing Amateur Astronomers Together

Introduction

The purpose of the Observer’s Challenge is to encourage the pursuit of visual observing. It’s open to everyone who’s interested, and if you’re able to contribute notes and/or drawings, we’ll be happy to include them in our monthly summary. Visual astronomy depends on what’s seen through the eyepiece. Not only does it satisfy an innate curiosity, but it allows the visual observer to discover the beauty and the wonderment of the night sky. Before photography, all observations depended on what astronomers saw in the eyepiece, and how they recorded their observations. This was done through notes and drawings, and that’s the tradition we’re stressing in the Observer’s Challenge. And for folks with an interest in astrophotography, your digital images and notes are just as welcome. The hope is that you’ll read through these reports and become inspired to take more time at the eyepiece, study each object, and look for those subtle details that you might never have noticed before.

This month’s target

Our object for the 150th monthly edition of the Observer’s Challenge is the tiny, but bright, planetary nebula NGC 6572, variously nicknamed the Emerald Nebula, the Blue Racquetball, and the Turquoise Orb. These names highlight the range of hues perceived by different observers. The nebula is young, perhaps only a few thousand years old. Its diminutive size led to its inclusion in some early star catalogs. NGC 6572 has a visual magnitude of 7.3, as determined by Stephen O’Meara, while its central star dimly shines at 13th magnitude. As with many planetary nebulae, published distances vary wildly. Values in the vicinity of 5000 light-years seem most likely. This pretty little gem was discovered in1825 by Wilhelm Struve.

NGC 6572 displays bipolar outflows in deep images. There’s evidence of interaction between the collimated outflows and the nebula’s elliptical shell. The interaction has broken up the elliptical shell such that parts of the shell have been accelerated, while the outflow has been slowed down and/or deflected. This supports the idea that such outflows are common in planetary nebulae and may play an important role in shaping nebular shells. https://ui.adsabs.harvard.edu/abs/1999ApJ…520..714M/abstract 

Mike McCabe: Observer from Massachusetts

The observer’s challenge object for July 2021 was NGC 6572, a bright planetary nebula in the constellation Ophiuchus, and I feel lucky to have gotten to have gotten an observation of it at all this month. Here in the northeast U.S. we’ve been experiencing record rains for this time of year, and clear nights have felt like a precious commodity. As luck would have it, on the exact night of the new moon we got a break and had a clear enough sky to do some observing. I put a scope out before dinner and waited for darkness.

The conditions were fairly typical of an early summer evening, with a high moisture content in the air lending to just fair transparency, but those same conditions also provided a somewhat steady state of seeing. On my sketch I noted the seeing as 2/5 early on, but further assessment as the evening wore on resulted in a rating of at least 3/5. And even though the target was quite bright by the observer’s challenge standards and I probably could have easily gotten away with a medium sized refractor, I chose the 10” F/5 Newtonian regardless. 

The target was a relatively easy star hop from the 3rd magnitude star Cebalrai, including a quick drop in on IC 4665, the “Summer Beehive” cluster, just because it was there. At low power the planetary nebula is virtually stellar, with very little evidence that you’re not looking at just a plain old star. Boosting the power up though begins to reveal the fuzzy discos nature of the object, and true to the form of many planetary nebula, magnification doesn’t seem to dim it excessively. Even though the dimensions listed in various resources shows the object to be somewhat elongated, it never appeared anything other than round to me.

With a variety of nicknames, including Blue Racquetball, Emerald Nebula, Green Nebula, and Turquoise Orb, I figured one more couldn’t hurt. I hereby dub thee “Fuzzy Blue Star”, because that’s exactly what it looked like to me. It’s well known that color interpretation is a highly individual thing, and greens are my weakest area in color vision. I feel a little like I’m missing out on something in all these astronomy observations, like the green in the coma of a comet, the green flash on the Sun, and now the emerald in this planetary nebula. But I fret not. I’m happy with the colors I see, even if it gives my wife fits when I tell her something is green, quite to the contrary of popular interpretation. She calls me colorblind. I tell her I see colors just fine. What does she know, anyway?

Glenn Chaple: Observer from Massachusetts

NGC 6572 – Planetary Nebula in Ophiuchus (Mag: 8.1, Size: 16” X 13”)

The visual observer is all too aware that, with the exception of double stars like gold and yellow Albireo and ruby-red carbon stars like R Leporis, the deep sky is a pretty colorless place. Bright planetary nebulae like this month’s Observer’s Challenge, NGC 6572 in Ophiuchus, are a notable exception.

NGC 6572 was discovered by the Russian-German astronomer Friedrich Georg Wilhelm von Struve in 1825. Struve was in the midst of a survey to catalog double stars when he came upon “a star surrounded by bright green ellipse of fuzzy light.” At the time, astronomers were unaware of the true nature of such a curiosity. Today we know that NGC 6572 is a planetary nebula – an expanding luminous shell of gas ejected by an aging star. It’s relatively young as planetary nebulae go, perhaps no more than 2600 years.

The 2000.0 coordinates for NGC 5672 are: R.A. 18h 12m 06.6s , Dec. +6° 51’ 13”. I star-hopped there by starting at the 5th magnitude star 71 Ophiuchi, the unlabeled star just south of 72 Ophiuchi on Finder Chart A. Finder Chart B shows an 8th magnitude star, SAO 123133 just northwest of 71 Ophiuchi. A line from this star through 71 Ophiuchi and extended 1.3° brought me to a triangle of 8th magnitude stars, NGC 6572 was a little less than a degree SSE of the southernmost star in the triangle.

At 39X in my 10-inch f/5 reflector, NGC 6572 appeared stellar. At 208X, it was definitely non-stellar when compared to a pair of stars immediately to its east, It seemed slightly elongated in a north-south orientation and was decidedly pale blue. I was unable to detect the central star, which is said to be 13th magnitude.

NGC 6572 is approximately 5000 light years away. This translates to an actual diameter of ⅓ light year.  

Jaakko Saloranta: Observer from Finland

July: NGC 6572 – Planetary Nebula –Ophiuchus

NGC 6572 with 10 inch GSO @ 625x (5′)

The observation and sketch was made from a suburban observing site with a naked eye limiting magnitude of ~5.8 near zenith and SQM-L reading of 19.80 from the same region. Telescope used was a 10 inch GSO telescope with multiple magnifications. Weather was fairly average: temperature in the low 50s, rising humidity towards midnight and some cirrus clouds starting to emerge at around 11.30 pm.

I felt I couldn’t get a clear view of the object – probably due to the average seeing conditions and somewhat low altitude of the object (32 degrees above the horizon). What I did manage to see was a “barely N-S elongated planetary with some structure in the middle. From time to time ring structure seems visible with some additional detail around it. No central star.” I could not discern any color from the object although I am familiar with the several – quite colorful nicknames – for this object.

Gregory Brannon: Observer from North Carolina

I went observing on the 11th. My cousin Quinn was with me, and I showed her M92 and M13, before deciding to try to find two planetary nebulae, Minkowski’s Footprint, and the Observer’s Challenge object for this month, NGC 6572. I was unable to find Minkowski’s Footprint after a while of trying, so I gave up for the time being and went for 6572.

By now, the clouds were coming across the sky in bands, obscuring and then revealing the part of the sky I was using in about equal measure. It took some time to star hop, using the screenshot from stellarium mobile as reference both mirrored and unmirrored. It became easier when I realized actually there was a dim but visible star right next to the object. At that point it was a race against time as a band of clouds, a band of clear, and a final neverending haze was approaching. The nebula appeared as a bright blue star at mid power (80x), nearly a point source, but with the weird averted vision behavior consistent with my experience of planetary nebulae. I scrambled to put a higher power in (I had already prepared the UHC filter), but in my haste I was a little too greedy with the magnification. At 428x it was not in the field of view. I pulled the Barlow out and put just the 7mm in (171x), and by then the cloud band was coming through. I lost it by the time the cloud band departed. Back at 80x, I located it again, switched to 7mm, and confirmed that it was a nebula. Very surface-bright and very noticeably pale-blue, very elongated (easily 2:1), and very small. I tried to scramble to go to the higher power (at that surface brightness I felt I could), but then the neverending haze arrived. I settled for a sketch from memory.

2021-July-12 10:40 PM EDTNGC 6572 – Planetary Nebula in Ophiuchus10″ f/5 Dobsonian, 2.5x Barlow, 7mm 58° eyepiece, 429x, UHC filterThe planetary nebula is a brilliant pale-blue at low power, its color and peculiar behavior under averted vision being the only thing which reveals its non-stellar nature. At 171x, it is noticeably elongated and yet retains significant surface brightness at a small size. It is almost stellar at first glance. At 429x, with the UHC filter, the blue coloration is even more dramatic and the nebula mecomes a bright blue ball with a slightly dimmer elliptical fringe elongated N/S, and slightly more sharply elongated in the North.

I also observed the object with the CPC800 (8″, 250x, no filter) belonging to the Cline Observatory, during a practice session on the Thursday before our re-opening to the public. It almost seemed to show a little better contrast though less saturation, and I felt it had a bit sharper edges and pointier ends. But this is the sort of thing your brain imagines. I got a brief impression out of observatory director Tom English–something to the effect of “nice color.” (Later: but not large or obvious enough to show during public nights.) Another observatory volunteer mistook it for a star at first.

Mario Motta: Observer from Massachusetts

NGC6572 is a very tiny object (16×12 arc seconds). Got this last week, poor night with some turbulence, with an H alpha, O3 , and S2 filters. Very short exposures as it is very bright. Visually a small “blue spot”.

Image attached, about 20 minutes each filter, O3 dominated…thus very blue. No detail that I can see. Only good image on line I found is by the Hubble, but can’t match that one! However, a nice object.

Rony De Laet: Observer from Belgium

NGC 6572 is a small but very bright planetary nebula. At 100x it looks like a swollen turquoise star. With each higher magnification, the object’s true nature becomes clear. The best views are at 400x and at 600x. No filter is needed. I see a bright elliptical nebula oriented north-south. It features a central disk of an even higher brightness. The eastern and western edges of the central disk are brighter. These edges seem to continue along the border of the elliptical nebula, forming an S-shape  The central star is not visible, but the central spot of the disk seems a little brighter. All these details are very subtle. It takes time and moments of good seeing to see them. Moving my attention from the centre of this planetary to its borders, I can’t neglect the outer nebulosity. Many observers consider this extended glow as the unavoidable glare of the bright planetary. I am not convinced. When I repeatedly move the planetary through the field of view, the extended halo becomes a well-defined physical part of the planetary nebula.

Site : Bekkevoort, Belgium (51° N)
Date : July 17, 2021
Time : around 23:50 UT
Telescope : Taurus 16”
EP: Morpheus 9mm 76°, 200x / 6.5mm 76°, 280x / 4.5mm 76°, 400x / Omegon 3mm 55°, 600x
Filter : none
Seeing : 4/5
Sky brightness : 20.2 magnitudes per square arc second near zenith (SQM reading).
Sketch Orientation: N up, W right.
Digital sketch made with Corel Paint Shop Pro X2, based on a raw pencil sketch.

Roger Ivester: Observer from North Carolina

NGC 6572 – Planetary Nebula in Ophiuchus 

Date:  May 2021 

Telescope:  6-inch f/6 Newtonian Reflector 

Eyepiece:  20mm + 2.8x BarlowSketch Magnification:  128x

NELM:  ~4.9 Magnitude

I knew that fine detail of this planetary would not be possible from my back yard, using a 10-inch reflector.  So, I chose to use a 6-inch f/6 reflector, mostly for convenience, but not really expecting much difference from the 10-inch.

With the 6-inch, this planetary is very small, mostly round and featureless, but with a pale bluish color.  

This is definitely a large telescope object for the visual observer.  

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