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Cosmic Challenge: Abell 12

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Cosmic Challenge: Abell 12


February 2024


Phil Harrington


This month's suggested aperture range:

10- to 14-inch (25-36cm) telescopes










Abell 12


06h 02.3m

+09° 39.3





Deep-sky objects can be challenging for several reasons. Some are especially faint, while others are especially small, and still others are so large that they can't fit into a single eyepiece field. Or the problem might be that a particular target is so close to another, noticeably brighter object that the light from that intruder all but obliterates the quarry.


The latter problem plagues planetary nebula Abell 12. It shines at about 14th magnitude, which is not exactly bright, but is also not exceptionally dim for a telescope 10 inches (25 cm) or more in aperture. The problem, however, is that it is located a scant arcminute away from 4th-magnitude Mu (μ) Orionis. That's why it's known by the nickname the Hidden Planetary.

Above: Evening star map. Credit: Map adapted from Star Watch by Phil Harrington

Above: Finder chart for this month's Cosmic Challenge.
Click on the chart to open a printable PDF version.


The nearness of the planetary to the star begs the question, "Are we looking at a former binary star system where one of its members is no longer with us?" According to data from the HIPPARCOS parallax-measuring satellite, we are not. Mu (μ) Orionis is 152 light years away. Abell 12 is some 6,900 light years from Earth. When we look toward this stellar odd couple, we are just looking at a chance alignment of two objects at quite different distances.


Were it not for that interloper, Abell 12 would undoubtedly have been discovered by William Herschel and included in the NGC. As it is, however, he and son John missed it during their intensive sky searches. Instead, George Abell was first to uncover this little bubble of expanding hydrogen gas while he searched the Palomar Observatory Sky Survey photographic plates in 1955. In the original image, Mu looked as though it was blowing a bubble while chewing gum. Only after more intensive study did Abell realize that the Bubble Gum Nebula, as I prefer to call it, was a separate object.


Mu (μ) Orionis is found 6° to the northeast of Betelgeuse [Alpha (α) Orionis] along the Hunter's raised arm, so Abell 12 is a snap to aim toward. To have any chance of seeing it, however, takes some effort. The light from that star, nearly 1,600 times more intense than the planetary, can completely overwhelm the tiny, 37" disk.


When all of these pre-existing conditions are met, point your telescope toward Mu with a reasonably high-power eyepiece in place. Experience shows that a magnification of 150x or more produces the best results. Experiment with different eyepieces as seeing conditions allow, but you will probably find that a narrowband (UHC-type) or O-III filter is a must regardless. Not only will the filter enhance the planetary, it will also help to muffle the star's light some in the process.


Above: Abell 12 looks like a faint reflect of the bright star Mu Orionis in this image taken by CN'er Brent Knight (BrentKnight). Visit his gallery for other great astroimages.




Above:  Abell 12 as seen through the author's 10-inch (25cm) reflector.


Under optimal conditions, Abell 12 shows a perfectly round disk with sharply defined edges, especially on the side facing away from the star. My notes recall the disk as appearing uniform in texture, although some other observers report a very subtle ring-like appearance. Abell 12 has been glimpsed through telescopes as small as 6 inches (15 cm) aperture, so even if you do not have a double-digit aperture, give this one a go. You just might be surprised.


Good luck with this month's Cosmic Challenge! And be sure to post your results in this column's discussion forum.


Until next month, remember that half of the fun is the thrill of the chase.  Game on!

About the Author:

Phil Harrington is a contributing editor to Astronomy magazine and is the author of 9 books on astronomy.  Visit www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2024 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.


  • Urban Observer likes this


David Knisely
Feb 02 2024 02:25 AM

The OIII filter in my 10 inch Newtonian under dark skies makes Abell 12 almost trivial to see, as it really knocks down the light from Mu Orionis while preserving the oxygen III emission of the nebula. 

    • PhilH, John O'Hara and Knasal like this
John O'Hara
Feb 02 2024 08:56 PM

What impresses me is that Phil observed these challenge objects from his suburban back yard with a NLM of no better than 5.  

    • PhilH and Knasal like this
Feb 03 2024 10:02 PM
I enjoyed the challenge with my 12 inch LB, it seemed to me that a O-lll was a must to glimpse it, but I did see it! Thanks Phil!
    • PhilH likes this
Alex Swartzinski
Feb 03 2024 11:24 PM

I just got in from observing this object under mid NELM 5 skies with a 15" dob.


With a UHC and 330x, the nebula was detectable as a round dim glow which is slightly brighter in the center. It was completely detached from the star halo at this power and fov. Using lower powers, Mu Orionis hides the planetary. I also attempted to observe this object unfiltered but I wasn't able to make any detections without the UHC.


I've been meaning to give Abell 12 an attempt for a long time and this challenge served as a great reminder every time I opened Cloudy Nights!  Thanks for the great challenges Phil. 

    • PhilH and Knasal like this

I was able to detect this planetary with my 10" under Bortle 6 skies, using my DGM NPB filter and high power. Here are my notes:

"This dim planetary appears very close to Mu Orionis, presenting as a round, featureless extension of the star's glare, situated at the 10-11:00 position at the time of writing. It is an averted vision-only object and would be invisible without the filter. I was able to spot it with the 12mm (148x), but my best view was with the 6.5mm (274x) and filter."

I honestly wasn't sure if I could see it with this level of light pollution, but I gave it a shot and was pleasantly surprised.

    • PhilH likes this

Another cosmic treasure! Thanks Phil!


I took a few images with a C8/Hyperstar, ASI678MC, NBZ II filter. It wasn't really visible in my Bortle 8 view without the filter.


Attached Image: Light_abell12_30.0s_Bin1_678MC_20.jpg

    • PhilH and Knasal like this



I tried something different last night and got additional detail: ASI533MC Pro, C11, Meade .33 reducer, UV/IR filter. 99 images, 30-seconds each for 50 minutes total. 


Some of Phil's targets are so special you can visit them again and again.



Attached Image: abell12.jpg

    • PhilH and Knasal like this
Feb 16 2024 12:16 PM
Nice thanks for sharing!
    • PhilH and Sky King like this

Thanks for sharing Phil. 


This is one of my favorite Abell PNes.  It was pretty easily seen with my 22".  One of the key things is to make sure that the optics are clean as any glare could wash out the Abell.  And don't be afraid to use magnification to create seperation.  It is only 1' from a 4th mag star!  So bump up the magnification to 300+.

    • Mark SW and micromaxcomputer like this
Feb 28 2024 03:18 PM

"Hidden" Planetary Nebula Abell 12 in Orion. Meade LX200 12" 0.63FR with Astronomik CCD CLS filter on ZWO ASI174mc cooled camera, 50 45sec images put thru DSS / PS


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