University Lowbrow Astronomers

Planetary Primer.

by Christopher Sarnecki
Printed in Reflections: January, 2006.

Chris Sarnecki

Chris Sarnecki using his special Planetary Nebula glasses - Wow, those are cool!

No Fear - Science tells us planetary nebulae are expanded shells of glowing gas shed by low mass stars moving in to their declining years. To beginning and intermediate observers, planetary nebulae are those pesky smallish orbs that are to be avoided with the exception of a few of the most brightest and easily found planetaries. The big and bright planetaries that we all are familiar are the four Messier planetaries M27, M57, M76, and M97. We are comfortable with these objects because; well, that is the way we are brought up to observe. Granted the four Messier planetaries are somewhat large and bright; but, as we will see, other New General Catalog (NGC) planetaries are also worthy of our attention. In this article, planetary nebulae will be listed, for your viewing convenience, as indicated below starting with the four Messier planetaries. Note the magnitude and size of the listed planetary. This is where we will gain the confidence to move on to NGC planetary observing.

Catalog No. Mag Size Coordinates - RA/Dec Common Name Remarks
M27 7.3 8’ x 4’ 19hx59.6m/+22^43’ Dumbbell Nebula BIG & BRIGHT
M57 8.8 86”x63” 18h53.6m/+33^02’ Ring Nebula less big & bright
M76 10.1 67” 01h42.4m/+51^34’ Little Dumbbell smaller, fainter
M97 9.7 2.8’ 11h14.8m/+55^01’ Owl Nebula *

* - Preachy Note - Again note the magnitude and size when observing planetaries.

Getting Started - Most of us use illuminated reticule finders such as a Rigel or Telrad, for general observing. When hunting down planetary Nebulae, it helps to use a finder scope to zero in on the planetary star field after using your illuminated reticule finder to find the general target area. I use an 8 x 52-mm right-angle, correct image finder scope. A straight through finder scope is also helpful. Without use of a finder scope, locating a smallish fainter planetary is probably going to be a chance occurrence. With a finder scope, you are pretty much guaranteed to locate the correct star field. I like the right-angle, correct image finder scope for a couple of reasons. The right angle finder scope eyepiece lets me get my head (and eye) in an easy position that straight through finder scopes don’t; and, the image seen in the correct image finder scope matches that of my star chart. Next get a decent star chart. I use Star Atlas 2000 by Wil Tirion and Roger Sinnott. You can use any star chart of your choosing, but you are going to need a star chart. Plan a short list of planetaries that are expected to be up during your observing run. Because this article is written in the summer, let’s assemble a short list of some easy Summer NGC planetaries.

Catalog No. Mag Size Coordinates - RA/Dec Common Name Remarks
NGC 6543 8.1 22”x19” 17h58.6m/+66^38’ Cat’s Eye Nebula > Bright as M57
NGC 6572 8.1 11” 18h12.1m/+06^51’ Green Nebula ** > Bright as M57
NGC 6826 8.8 28”x25” 19h44.8m/+50^32’ Blinking Planetary > Bright as M57
NGC 6818 9.3 22”x15” 19h44.0m/-14^09’ Little Gem < Bright as M97
NGC 7009 8.3 28” 21H04.2m/-11^22’ Saturn Nebula > Bright as M57

** - Nicknamed by Lowbrows just after observing it. Catalogs indicate as Emerald Nebula, Blue Racquetball, Turquoise Orb.

The listed planetaries above are not at large as M57, but are with the exception of the Little Gem, they are as bright as or slightly brighter than M57. Smaller and brighter planetaries will have a brighter per arc-second area when compared to an object such as M57. This means these objects are going to appear brighter, and perhaps easier to find. This should give you the confidence to move on to more challenging planetaries.

Intermission - Wisconsin Micro Brews, Pt 1

Challenge your self - Hopefully by now you are thinking this all seems possible. I can do this. If so, then maybe your astronomical world can open up to these intriguing challenge objects.

Catalog No. Mag Size Coordinates - RA/Dec Common Name Remarks
NGC 6210 8.8 16” 16h44.5m/+23^49’ Turtle Nebula It’s Green!
Yea, it’s small, but it is relatively bright. The fact that it’s green makes it a standout on the star field and easy to find at lower power.
NGC 6369 11.4 38” 17h29.3m/-23^46’ Little Ghost Nebula Easy
This one is faint, but easily located because it is amongst the three naked eye stars 51, 44, and 42 Ophiuchus. This small star asterism is located between the tea pot of Sagittarius and Antares in Scorpius. It was an easy target in the Club’s Cave 8 inch f6 scope. This one will give you the confidence to seek out other fainter planetaries.
NGC 7293 7.3 16’x12’ 22h29.6m/-20^48’ Helix Nebula BIG, 300 Lt Yrs
How did this massive object qualify as a challenge object? True this object is big and has a bright magnitude, but the Helix Nebula has low surface brightness and located in a southern nondescript area of the sky. Just locating this object will improve your star hopping ability. It is much easier to observe at a dark sky site than it is at Peach Mountain.
NGC 6445 11.2 33” 17h49.3m/-20^01’
Now it gets a little more difficult, but we will be rewarded for our efforts. This object shares a low power field with the 9th magnitude Globular Cluster NGC 6440. It is quite a pretty sight. I encourage you to check this one out.
IC 1295 12.5 1.7’x1.4’ 18h54.6m/-08^50’
Save this one for a dark site such as Lake Hudson or Black Forest. This planetary also shares a low power field with a Globular Cluster. NGC 6712 is an 8.1 magnitude object that sits about two degrees east northeast of the open star cluster M 26. I have tried a couple of times to see this faint and large planetary at Peach Mountain, but didn’t really see it until looking for it at Lake Hudson. The key to finding this planetary is to know where to look. Check out the fine article by Sue French in the August 2005 issue of Sky & Telescope about this planetary nebula and where to look for it.

What to Look For - When looking for planetary nebulae in the low power field of your telescope, remember these objects look like out of focus stars with greenish-bluish disks for the brighter ones. Higher magnification turns these fuzzy looking stars into resolvable disks. Observing some planetaries will reveal the white dwarf central star. Some nebulae look like mini versions for the famous Ring Nebula, M57. Some even have discernable shapes beyond the usual circular disk. The Saturn Nebula, NGC 7009 has an oblong shape that resembles a low power view of the planet Saturn.

A technique to help locate smaller planetary discs from their stellar neighbors is to hold an Oxygen III filter up to the eyepiece. Planetary nebulae respond well to OIII filters while many of the fainter stars will drop out of the view. In my experience the OIII filter overpowers some the fainter features of the nebula. So I typically will not screw the filter into the eyepiece, but hold it over the eyepiece. Once I find the nebula in question, I put the OIII filter down and enjoy the view without it.

Once you master the bigger and brighter planetaries listed in this article, you should have the confidence to start looking for those “pesky smallish orbs” that you use to avoid. Small planetaries will require more magnification, but you will be rewarded with finding objects that you use to pass by. Let me know how your planetary observing runs go next time you see me on the “Hill.” I’ll be the guy chasing down yet another planetary. [OBTW - Don’t forget to note the magnitude and size.]


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