Object of the Week July 20, 2014 NGC 6572 The Curious Tale of...

[deepskytraveler]

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. 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.

ngc6572-blue-racquetball-sketch.jpg

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.

Ngc6572-hst-R656G502B487.jpg
6170514-OPH_NGC 6572_13in_220X (Medium).jpg

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!