Planetary Nebula Observing at TMSP

By Matt Vartanian

I went up to the Table Mountain Star Party this year with an observing program that mostly consisted of planetary nebulae. Aside from the long drive, bugs, clouds, rain, overcrowding and vehicle problems, this year's TMSP was ideal.

On my observing list were 10 non-Messier Planetary Nebulae, half of which were new to me. Based on the views in my 16" scope I would expect all of these to be worthwhile targets in an 8" scope and most would be fine targets in much smaller scopes. These planetaries were fairly easy to find using only a single chart printed from SkyMap Pro for each object. All were readily recognizable as non-stellar objects at 92x, but planetaries really start to show off at 200x and above. I used filters on these objects, and although they work well I found that the unfiltered views were quite nice. In fact some of the planetaries did not benefit at all from the use of a UHC or OIII filter.

For those who are not familiar with this class of object, the term Planetary Nebula was coined by William Herschel. He used this term because many of these objects looked like the planet Uranus that he had discovered earlier in his career. As planetary nebulae were discovered they were added to a variety of catalogs. In 1967 Perek and Kohoutek compiled all known planetaries into the PK catalog, which was dedicated exclusively to planetary nebulae. The format of this catalog is the prefix PK followed by the galactic coordinates of the planetary to the nearest degree, and an ordinal number if more than one planetary is found at these coordinates. Around ten years ago the IAU recommended yet another Planetary catalog using PN G as a prefix. This catalog is even more comprehensive and provides cross-references to other catalogs. The format is almost the same as the PK catalog but adds another digit of accuracy to the coordinates. This is the catalog used in Uranometria 2000.

Planetary Nebula formation is the most common evolutionary path for stars (up to 8 solar masses). Late in its life a star expels shells of gas, then shrinks down to a white dwarf. During this contraction its temperature increases dramatically generating high-energy radiation. The high-energy UV radiation coming from the star excites the gas shells causing them to fluoresce, producing the glowing planetary nebula that we observe. This description is, of course, an oversimplification of a much more complex process. The glowing gas shells continue outward very rapidly, diffusing in a relatively short time of less than 50,000 years. This is why planetary nebulae, although the most common evolutionary path for stars, are not as common an object as one would expect. There are, in fact, less than 1500 cataloged planetaries.

So without further delay, here is my observing list with brief descriptions of what I saw:

• NGC 7009 in Aquarius
  The Saturn Nebula, greenish oval that looks a little like Saturn due to material ejected in opposite directions from the star. These ejecta are called ansae or FLIERs. There is currently no consensus on how they are formed.
  NGC 6543 in Draco
  Cat's Eye Nebula. Obvious bright blue-green shell surrounding visible central star forms a "cat's eye." Shell shows bright regions near the edges of the "eye." A favorite of mine.
• NGC 6826 in Cygnus
  With direct vision shows a star with little nebulosity, with averted vision shows the PN as a disk, hence the common name The Blinking Planetary.
• NGC 40 in Cepheus
  Central star visible surrounded by halo that has a mottled characteristic. Dark region immediately surrounding the central star.
• NGC 6210 in Hercules
  Very small planetary with a striking blue color. Bright.
• NGC 6572 in Ophiuchus
  Appeared very much like Uranus, green hue and all. Strong color contrast with 6210.
• NGC 6369 in Ophiuchus
  Looks like a smaller version of M57. The ring itself had a dim side and a bright side and overall appeared translucent.
• NGC 6781 in Aquila
  Another ghostly object similar to M97 in appearance. Filter helps with this one.
• NGC 6818 in Sagittarius
  Bright medium sized disk with irregular edge on one side.
• NGC 6445 in Sagittarius
  Looks like a marshmallow pinched in the middle, or maybe a crushed beer can.

Planetary Nebulae have a couple of good things going for them. First, they are among the few deep sky objects that show color. And second, they have wonderfully creative names like The Blue Snowball, The Bug, The Cat's Eye, the Helix, and The Phantom Streak. But the most interesting thing about observing this list was seeing the incredible variety of structure that these objects possess. If you have not ventured past planetaries on Messier's list, you have a treat awaiting you regardless of your scope size.

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