Object of the Week February 7th 2016: Abell 33, The "Engagement Ring" Planetary Nebula
Abell 33
PK 238 +34.1
Hydra
RA 09h 39m 09.2s
Dec -02 48 35
Planetary Nebula
Magnitude 13.9
Diameter 4.5 arc-minutes
The night sky is full of jewels. Open clusters such as the Pleiades, the Double Cluster in Perseus, or NGC 2362 in Canis Major, have rightfully been named “jewels of the sky”, and there is a “Jewel Box” cluster (NGC 4755) in the deep southern skies. There is even an “engagement ring asterism with Polaris as the “diamaond”. Yet, to my eyes, none of all these compares to the beautiful “engagement ring” image conveyed by Abell 33, a big planetary nebula in the constellation of Hydra. Especially in photographs, it is one of the most striking images in the night skies. At the same time, it is attractive enough for visual observers as well.
This large planetary (almost 4.5 arc-minutes across) is located 2700 light years away near the celestial equator in the constellation Hydra, thus it is accessible from both northern and southern localities (except the far north and south). Abell 33, also known as PK 238+34.1 has a magnitude 13.4, which makes it a very low surface brightness object, with a 15.5 magnitude central star. Yet, as we shall soon see, it is not as difficult an object as it sounds. The main problem with observing this planetary is the 7th magnitude star HD 83535, which is located right at the edge of the planetary (in our line of sight of course!). At the same time, this very star assumes the role of the sparkling “diamond” in the “engagement ring” and makes this object such an attractive quarry for both the observer and the photographer.
My own interest in this planetary began after reading an article on “The best Abell Planetaries” by Eric Honeycutt in the May 2002 issue of Sky and Telescope. The photo on page 100 of the article certainly does not make justice to the appearance of Abell 33 in more recent photos that I have seen, but at the same time the rather gloomy appearance of the object somehow captivated me. I remember attempting it with the 10” equatorially mounted reflector I owned at the time but I failed to see it.
Ab33.jpg
Abell 33 from the Deep Sky Survey
My next attempt on this planetary occurred in the spring of 2009, this time using a 12” dobsonian from a mountain dark location. I had prepared beforehand a locator chart made from the Deep Sky Survey images, showing that the planetary was located right next to the middle star in a line of three 7th magnitude stars. To my surprise, using an OIII filter on a 32mm Plossl eyepiece I was able to detect an ill-defined circle of faint light touching the aforementioned 7th magnitude star. I could hold this faint circle of light using both averted and direct vision. Yet, with direct vision it remained an extremely faint ring of light, while with averted vision I could detect that the inside part of the circle was (just barely) brighter than the outer space around the nebula (somewhat like a huge diffused version of the Ring nebula). Needless to say, the nebula was invisible without the filter.
The not-so-exciting appearance of Abell 33 in my old 12” pushed it out of my observing projects for a few years, until I happened to stumble across some of the fine recent photos of the planetary, such as this photo by the ESO astronomers (http://scitechdaily.com/esos-large-t...bula-abell-33/). By the time I discovered these photos I had upgraded to my current scope, and my 18” Obsession definitely helped me to finally enjoy the subtle beauty of this planetary.
ESO-Views-Planetary-Nebula-Abell-33.jpg
Under dark skies, using an OIII filter on my 31mm Nagler, I was stunned to see a perfectly round sphere of subtle light touching the bright star. But the best view was obtained using my 13mm Ethos. The edges of the nebula became much better defined, and there seemed to be slight variations in the light visible within the sphere, the eastern half slightly brighter than the western. Also, the eastern edge of the nebula appeared distinctly brighter. This appearance did indeed remind me of the “engagement ring” appearance I had seen in recent photos of the object. By placing the 7th magnitude star just outside the field I could more certainly detect the subtle variations in the brightness inside the sphere (albeit with the loss of the “engagement ring” effect). I could detect a very faint star near the center of the nebula but I am not sure whether it was the central star.
Ab33a1.jpg
Removing the OIII filter almost eliminated the subtle appearance of the nebula. It was barely detectable as a faint patch using a 31mm Nagler. By increasing the magnification with the 13mm Ethos I could no longer detect the subtle patch of light, but I could see (using averted vision) the eastern edge of the nebula. Also, by removing the filter I could detect a few more very faint stars in the area of the sphere that were not seen using the OIII filter, one of which was presumably the central star.
Searching through the internet I found several reports stating that PK 238 +34.1 was seen through apertures as small as 8 inches. So, I decided last week to attempt this planetary through my 6” achromatic refractor. Using it at low power with and without filters I could see no hint of the nebula. Increasing the magnification to 67X using the Ethos 13, and by adding my trusty OIII filter, I managed to detect a barely visible patch of diffuse faint light at the correct position. It was very ghostly, and I could not even define its shape, it was just a patch of light. With averted vision it became slightly more defined, sort of round, but still it was a borderline observation.
Ab33SixE13a1.jpg
Using my 120mm achromatic refractor nothing was seen, with or without filter. So under my good (but not perfect) skies, this planetary requires at least six inches of aperture just to be detected, and I would say at least 10 inches to really squeeze some detail. But, the “engagement ring” appearance needs several inches. It was not seen as such with 12 inches, but it was evident with 18, so my guess would be that at least 15 inches are required. On the other hand, under extremely dark and clear skies, Abell 33 might be visible in even four inches, and maybe the “engagement ring” might show in lesser apertures.
So, next time you are out under a nice dark winter sky, turn your scope to Abell 33, and you might be pleasantly surprised. And, as always,
GIVE IT A GO AND LET US KNOW!
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