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deepskytraveler
July 21st, 2014, 05:14 AM
Object of the Week July 20, 2014 NGC 6572
The Curious Tale of the Green Emerald, the Blue Racquetball, and the Turquoise Orb

NGC 6572, PN G034.6+11.8, PK 034+11.1
Planetary Nebula
RA: 18h 12m 6.4s Dec: +06° 51’ 12″
Size: 15”
Mag: 8.0v
Mean Surface Brightness 13.6 Mag/arcsec2

Central Star
Mag: 11.0 (maximum)

My interest in NGC 6572 was first piqued by a brief reference to it as the Emerald Nebula in this 2012 thread on Deep Sky Forum: Color in DSO Objects (http://www.deepskyforum.com/showthread.php?275-Color-in-DSO-objects). I added it to my list of objects to someday research further and observe.

NGC 6572 is a small bright planetary nebula of high surface brightness, which makes it a good target for smaller scopes and high magnification. It boasts an integrated magnitude brighter than the Ring Nebula packed into a tiny 15" disk - resulting in a surface brightness 100 times that of the Ring! I found it visible in a 90mm (3.5”) scope under less than ideal suburban observing conditions. At low magnification it appears star-like, but with a very pronounced green color. Located in Ophiuchus NGC 6572 is well placed for most Northern Hemisphere observers, transiting around local midnight during July.

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Friedrich Georg Wilhelm von Struve discovered NGC 6527 in 1825. Struve, Director of the Dorpat Observatory at that time, is best known for his research on double stars and geodesy. In his 1827 Catalogus Novus Stellarium Duplicium (Catalog of Double and Multiple Stars) Struve also gives a table of the 9 “nebulae” he discovered, 4 of them original discoveries, of which NGC 6572 is one of the latter. In his discovery notes he calls it one of the “most curious objects in heavens.”

Let’s briefly explore NGC 6572. As previously noted with its tiny disk this planetary is easily mistaken for a star if lower magnifications are used. Yet apply some power and it becomes a “beautiful gem” (W.S.Houston). In 16”/18” scopes at 250x, “The planetary’s disk is slightly oval, about 18” x 15” N-S, and has diffuse edges but no central brightening. The central star is intermittently visible.” (Kepple & Sanner). A HST image shows the intricate details throughout the extended planetary, while this sketch (S.Coe) through an 13” scope at 220x is more how we will see it at high powers. As often found in a planetary the central star is a Wolf-Rayet star. In this case the star is also a variable, spectroscopic binary, reaching a maximum brightness of 11.0.

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No surprises until we dig into The Curious Tale of the Green Emerald, the Blue Racquetball, and the Turquoise Orb. So why are we now discussing 3 different objects? We’re not! Amazingly they are all one of the same, our Object of the Week - NGC 6572. Taking a look at the color a few notable observers have reported demonstrates why there are 3 common names.


“My old 10-inch reflector showed the vivid green color of this object with any power more than 50x. It is interesting to note that older observers have describe NGC 6572 as green, while the younger ones tend to call it vivid blue.” (Walter Scott Houston)

“I was stunned when I saw this one at 83x [18” dob]. In 25 years of observing with all manner of telescopes, I had never seen blue in a planetary nebula.” (Greg Crinklaw)

“Through an 8-inch refractor, NGC 6572 seemed distinctly robin’s egg blue. In a 10-inch reflector, the planetary looked blue green… With a 14.5-inch one night, it seemed greenish, while another night, it appeared strikingly turquoise with a 15-inch.” (Sue French)
“I first viewed NGC6572 through this 9.25" SCT and at lower magnification, it looked like a fuzzy blue star. At 235x, a very bright, striking blue (cyan) …” (Todd Gross)

“In every scope I have ever owned, from an 8" to an 18" this is the greenest nebula I have ever seen! This guy is as green as an Irishman's coat on St. Patrick's Day. Alright, alright, it is as green as lime Jello.” (Steve Coe)



I chased observing reports from books and the web and compiled metrics on the color of NGC 6572 as reported by each observer. Although neither scientific nor statistically significant, here are my initial findings.


Green 57%
Blue 22%
Blue-Green 14%
Turquoise 7%


Now it is your turn to contribute to The Curious Tale of the Green Emerald, the Blue Racquetball, and the Turquoise Orb. Post your observing reports and sketches as always. I encourage everyone to “give it a go” – get out with your scope, small or large and observe NGC 6572, it’s easy – and then “let us know” – post your color report. Be sure to include some basic information about you – the observer, your location, sky and weather conditions, and your scope – aperture, power, and any other information do consider relevant. If I receive a large enough response I will compile a more rigorous analysis for Deep Sky Forum.

“GIVE IT A GO AND LET US KNOW”
GOOD LUCK AND GREAT VIEWING!

Steve Gottlieb
July 21st, 2014, 06:14 AM
I've found you can have it both ways. Here's an experiment I've repeated with my 18". Maybe it's just me.

"At 225x I noticed an interesting color effect; although the color was a quite prominent bluish-green, while staring at the center the planetary decreased in size and the color changed to a deep emerald green."

Perhaps Jimi will comment about a completely different color that's also visible in NGC 6572 ;-)

Uwe Glahn
July 21st, 2014, 03:18 PM
Very interesting object Mark,

I remember a huge thread in the German forum astrotreff (http://www.astrotreff.de/topic.asp?ARCHIVE=true&TOPIC_ID=43843) (for translating use for example google (https://translate.google.com/)). The results were very different with no systematic or relation between aperture, sky quality and used exit pupil. My experiences were also very different with that object and I really don't know why. Reiner V. spoke about different sensitivity of the cone cells of each person but in my opinion it doesn't explain why the same observer sees different colors.

For the colors:
4" f/9; AP 2,2mm; NELM 4,5mag - light blue
10" f/10; AP 1,4mm-4mm; NELM 4,5mag - blue
20" f/4,3; AP 3,3mm-8,1mm; NELM 6,6mag - intensive turquoise to green with tendency to green (no blueish tone)
27" f/4,2; AP 1,2mm-6,5mm, NELM 6,5mag - intensive turquoise to green (no blueish tone)

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sketch: 27" f/4,2; 837x; no filter; NELM 6,5mag; Seeing II
http://www.deepsky-visuell.de/Zeichnungen/NGC6572.jpg

The CS was not visible without filter. With central star filter the star was visible without problems

Dragan
July 21st, 2014, 07:02 PM
Great object Mark! I looked at this object just the other night with Dave Kriege and his 25" from his observatory. In his scope @ 242x, the PN exhibited a striking green/emerald hue. It was very apparent. Conditions were rather benign with ok seeing but hazy skies and a real lack of transparency. I didn't SQM the skies but I'd bet they hovered around 21.00......maybe? (and that's being optimistic)

I do have more notes on this object from my own observations but being at work at the moment, I don't have access to my notes.

Great info Mark!

KidOrion
July 22nd, 2014, 07:42 AM
I can finally contribute to one of these!

Observed this one for the first time tonight with mediocre seeing, poor transparency, and ground fog rolling in. NELM 5.4. Used a 12.5" f/5 Dob and a 14mm 82* eyepiece (113x; the seeing wouldn't support higher powers much, although I did use a 6mm Radian [264x] on Merrill 2-1 earlier in the evening, to poor effect). Fellow AASI-er Harry Treece also observed it, with a C-11 (not sure what EP).

My notes: "Harry sees this one as obviously blue, but it seems greenish to me; not nearly as green as the Saturn Nebula, but with obvious color. Very small, maybe 10", and bright even with no filter. Nebula lies off the nose of an asterism that looks like a 'smashed Big Dipper,' with the bowl of the 'Dipper' to the F side of the nebula. Edges are fairly sharp w/o filter; with OIII, edges become 'cottony,' resembling effect of condensation on optics."

[1] The "condensation" comparison came from looking at the nebula with Harry's C-11 and--given the amount of fog creeping in--wondering if his corrector plate had dewed over. He had higher power and a nebula filter of unknown type (completely unmarked; the box said 'Nebular Filter', but I suspect it to be a narrowband UHC-type), and the nebula strongly resembled Mark's sketch.

[2] I'm pretty strongly colorblind in the red end of the spectrum, but thought my green/blue was OK. I wonder now, especially as the green I saw in the nebula wasn't nearly as striking as others have noted.

Jim Chandler
July 22nd, 2014, 02:59 PM
For the colors:
4" f/9; AP 2,2mm; NELM 4,5mag - light blue
10" f/10; AP 1,4mm-4mm; NELM 4,5mag - blue
20" f/4,3; AP 3,3mm-8,1mm; NELM 6,6mag - intensive turquoise to green with tendency to green (no blueish tone)
27" f/4,2; AP 1,2mm-6,5mm, NELM 6,5mag - intensive turquoise to green (no blueish tone)


That's an interesting progression of color vs. aperture. I looked up my log notes for 6572 and found an entry from August 2006 using the 82" at McDonald (perhaps you have heard of it). It was a good night, and we were using 1297x. Even I, endowed with weak color vision, saw it as blue. Some of our group saw the object as blue with a purple halo around it.

Howard B
July 24th, 2014, 01:15 AM
Here are the various descriptions of color I've written in my notes for 6572. All are separate observations, starting in 1992, and were made with my lowest available magnification, which ranged from 83x to 106x:

No color mentioned (20" f5)
Bright Green (20" f5)
Bright greenish-blue (20" f5)
Robin's egg blue (28" f4)
Jade (28" f4)

These color descriptions span 22 years, but since I've listed them in chronological order they don't suggest I'm loosing color sensitivity, thank goodness. But the difference from bright green to Robin's egg blue does suggest that observing conditions play a role in how I've perceived 6572's color.

Bgman
July 26th, 2014, 12:11 PM
Hey everyone,

I had a look Thursday night with my 25" at this for the first time that I can recall. My skies were turbulent and the transparency was not as good as was predicted for the night. I saw it as a beautiful green color at 264x. I really enjoyed the view and will revisit this one again in better conditions!

hajuem
August 7th, 2014, 10:12 AM
Hi
I have a current observation with 16 inch
Seeing was excellent, Fst 6m5,
1327
v = 1040 without filter

I could zoom with a 2x Barlow to 1040x and I saw an oval, elongated object with a bright central region and indicated light "shells" in the southern and northern area.

CS Hajü
www.astromerk.de

Marko
October 6th, 2014, 02:31 AM
I'm going to not discuss the color as to me it was like has been said, striking teal 'emerald' in the sky.

While I was observing this in near ideal 8000 foot skies in my 18" f/3.7 using 6mmDelos or 282x last July 22 I became puzzled by a very large mottled glow first noticed east of the object and extending out about 15'.

After being puzzled and not able to convince myself this was my imagination I discussed this with Jimi who was present and he said there was some sort of huge outer shell. I went back and tried to find the shell in other locations and although much dimmer the drop off of mottled very dim area to background black was noted in this observation. I have yet to have found any deep space photo that confirms my observation yet I don't think it was imagined, it drifted with the stars and scope movement made the edges of it very obvious to the east but less so in the dimmer other outskirts.

Here is my OR segment [notes in brackets are a number or direction translated from my relative direct observing notes]
con: Oph fav: 5 cmt1: BlueRacquetball:
NGC 6572 PK 34+11.1 PNG 34.6+11.8
Plan 9.0p 11.0" 13.1 2a STE 18 12 06.4 +06 51 11
12:20am 6mmDe 282x 0.7unit [1.6'] elongated same dir as two unit length stars [NS] 8mmDe 211x unfiltered shows 1/2 unit [1.15']. Very obvious teal glow of nebula with no filter. 6mmDe 282x with NPB filter on this tiny object we see perhaps 0.7 unit size [1.6'] elongated same as the two unit length stars [NS]. Using NPB filter 6mmDE I see upper right [E] of the field honeycombed with blotchy light/dark of 1/30fov sort of spot size going out 1.2fov [18']. This glow is strongest up and to the right. Jimi then told me this was a huge shell so I will carefully look for it all around. I see dimmer glow than from up and to right with only This glow maintains dark/light mottling as it moves with drift. As unit of measure [2.3'] is to use two lower right of 3 stars pointing to 4:30 in a line of 3 that is up and to right of the object but still in the field. North is mostly down to 5pm. Down and to right 1fov [15'], and also up and to left. Down and to left it is 3/4fov [11.2']

Steve Gottlieb
October 6th, 2014, 09:59 PM
There's an interesting discussion of a possible outer halo to NGC 6572 in Stephen O'Meara's book "Hidden Treasures" with quotes from Jay McNeil

You can read the section here (http://books.google.com/books?id=a6VY0Q1zsJoC&pg=PA449&lpg=PA449&dq=%22NGC+6572%22+outer+halo&source=bl&ots=9Q3hZ3YxZd&sig=bBV_8rmHOrgxC5GR4KjGPHYYtxU&hl=en&sa=X&ei=Hg4zVMyKFOSCjALskYGwAQ&ved=0CEEQ6AEwCA#v=onepage&q=%22NGC%206572%22%20outer%20halo&f=false)