A difficult catch -- Abell 35!

Also known as Sh2-313 in the Sharpless catalog, this is a low-surface-brightness planetary nebula. From out perspective on Earth, it's about the same width is the main part of the Helix Nebula, but it is a lot older and more distant. It has a surface brightness of 26.2 mag/arcsec^2, which is far dimmer than the level of light pollution where I lived in the Bay Area of 18.5 mag/arcsec^2, and even dimmer than the sky brightness at a truly dark sky site. However, thanks to narrowband imaging and stacking, I was just barely able to pull it out from the background noise!

My Aussie friend Terry Robison has a nice writeup about it on his AstroBin:

"At the core, what appears to be a very bright star is a binary star system responsible for creating the surrounding nebula. Spectroscopic measurements reveal the pair are rotating very quickly, almost enough to tear themselves apart. One of the stars is a white dwarf. The incredible forces generated by the rotation on the white dwarf has shed away its outer layers thousands of years ago. This has produced the surrounding planetary nebula we can see today. The glow is caused by the UV radiation of the white dwarf ionizing the expanding shell of material as it expands into space.

The bow shock is due to the motion of the nebula and its central star moving through the gas between the stars. It’s fairly rare to look at stellar objects and see features that suggest motion. I have imaged a few other planetary nebula that have features highlighting their movement through the interstellar medium. This can be represented with wave like structures, localised brightening of gaseous regions, or distortions. One of my favourite objects showing this phenomena is the Skull Nebula.

Abell 35 is located in the constellation Hydra. To throw some perspective on this, the full moon is about 31 arc-min across, and SH2-313 is 10 arc-min across. Abell first identified this planetary nebula in 1966. It is possibly the largest PN known at 1.6 pc diameter, and is about 360 pc distant (Jacoby 1981). It is also the oldest PN known (Bohuski 1972)."

I'll need to collect more images on it!