Fascinating target. This nebula is very old, nearing the end of its existence. Discovered in 1982 by Heckathorn, Fessen and Gull.
This is type F planetary nebula- its interior is evenly filled with gas. Its source is a binary- V664Cas, where one of the stars is a white subdwarf (sdO). It's very small- about 0,5 Solar mass, but extremely hot- 83 000K. The second star is still in its main sequence, with mass of 1.1 Solar mass, radius of 1,3 Solar and surface temperature of ~5 500K. Both stars orbit the center of mass very rapidly- every 14 hours and thus are very close to each other (a few million kilometers).
Why does this nebula look the way it does? When the progenitor of the sdO star formed the planetary nebula, the entire system was traveling through interstellar Hydrogen medium rapidly- supersonically in respect to it. That resulted in formation of the collimated hydrogen tail and a complex set of shockwaves in the "head"- one traveling outwards that subsequently slows down to below the speed of sound in the medium. What follows is a termination shock, traveling inwards, compressing the gas bubble surrounding the binary and forming the peculiar gap between the inner and outer layers.
Here's a great paper explaining this in detail:
Modelling the cometary structure of the planetary nebula HFG1 based on the evolution of its binary central star V664 Cas. A. Chiotellis, P. Boumis, N. Nanouris, J. Meaburn, G. Dimitriadis; Monthly Notices of the Royal Astronomical Society, Volume 457, Issue 1, 21 March 2016.
Technical notes:
I consider this to be my first "big boy astrophoto". Quite pleased with it, but it could use 10-20 more hours of OIII.
Workflow
The two most challenging steps were stretching and noise reduction. SNR on this thing is exceptionally low, despite investing ~65 hours into this project. However, when there's a challenge there's also a reward.A. Pixinsight:
- DynamicCrop,
- DynamicBackgroundExtraction,
- NoiseXterminator,
- StarXterminator,
- GeneralizedHyperbolicStretch + NoiseXterminator.
B. Photoshop:
- Gradient Map – red on Ha, blue on OIII.
- SelectiveColor, Levels, CameraRAW, etc.
- Added RGB stars.
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