Abell 39 Planetary Nebula in Broadband and Narrowband O (Hercules), June 23,24 2023
Planewave CDK12.5 - ASI6200MM
A-P 1100 AE GTO, Antlia Pro BB and 3nm NB filters
L: (28 x 150s exposures, Bin 2x2, Gain 100)
R,G,B: (20, 20, 20 x 180s exposures, Bin 2x2, Gain 100)
O: (25 x 540s exposures, Bin 2x2, Gain 200)
Total Integration Time = 7.9 hours

Abell 39 is a wonderfully spherical planetary nebula in Hercules. It is quite remarkable that it sphericity has been maintained given its huge (> 1 light-year) diameter. It’s central white dwarf star is only slightly off centre. My camera catches the PN as a somewhat fuzzy sphere with some surface brightness variations that give it “lunar” or even organic-cellular type appearance. The dominant visible light from planetary nebula is the blue/green spectral line given off by highly excited O++ ions when in an ultra-low density state, which makes this planetary nebula’s colour green to blue, depending on the whim of the image processor. Little hydrogen signal is given off, although the nebula did also yield a signal in broadband red and I didn’t try a Ha filter. The surface brightness is fairly strong, and its size is reasonably large making it a nice, fairly straighforward target for imaging.
The other interesting feature of this planetary nebula is that it is quite transparent. I counted over 30 galaxies readily identifiable in the wider image, and 2 of these galaxies can be seen through the nebula itself. Often we can’t easily tell if a star is in front of, or behind nebulosity. In this case, we know these galaxies are actually behind. It is the only case I can recall where we can see galaxies through a planetary nebula.
In total (nebula and central star), the mass is roughly 1.2 (20% greater than) our sun and is 7000 l-y distant.