NGC6992 - The Eastern Veil, Take II

Object Description: (Credit, with thanks, goes to Steven James O'Mera, as written in the book Deep Sky Companions)
About 15,000 years ago an unknown star died in a spectacular fashion in the constellation of Cygnus, the Swan.  The event, a Type II supernova explosion, would have briefly dominated the night sky on Earth.  If prehistoric peoples were in the habit of gazing toward the heavens for meaning, they might have found it in the sudden appearance of this "new" star, which could have rivaled the full Moon in brilliance and remained visible for weeks in broad daylight.  The creators of the oldest known human art, the Cro-Magnon, might have witnessed this stellar apparition, and conceivably they recorded the event in a cave painting somewhere - though if they did, the artwork remains undiscovered.  

What is captured here is the remnant of that powerful blast.  Popularly known as the Veil Nebula, NGC 6960, NGC 6992, and NGC 6995 together constitute the corpse of a single supergiant star that perished in that supernova explosion.  Also known collectively as the Cygnus Loop, its various pieces are parts of an expanding shell of dimly glowing gas measuring six Moon diameters in our sky, or 80 light-years in space.  It appears loop-like in part because we see more gas in a give square arc minute of sky when we look at the shell's edges than when we gaze through its middle.  The Veil Nebula complex is thought to be between 1,300 and 1,500 light years away.

A Type II supernova announces the death of a supergiant star - one that starts life with at least 8 times the mass of the Sun.  Such stars age about 1,000 times faster than does our Sun.  As it ages a supergiant star forges ever-heavier atomic nuclei, from carbon early in its life to iron near the end.  Each nuclear-fusion stage releases energy and helps the star fight the ever-present pull of gravity.  Once the star's core creates iron, however, nuclear fusion can proceed there no further. (Energy is consumed, rather than generated, by the fusion of elements heavier than iron.) Since energy is no longer being released, the outward pressure that supported the star stops.  The star succumbs to the force of gravity, and within seconds the core of the once-mighty supergiant collapses into a sphere only 10km across.  The rest of the star rushes inward, only to rebound off that dense core.  A quarter-second later, the star ends its life in a cataclysmic explosion that will shine with the light of a billion Suns - the luminosity of a small galaxy.  The blast hurls several solar masses of material outward at speeds measured in thousands of km per second.  Plowing through space, the ejecta collides with interstellar material, heating it up and causing it to glow.  The ejecta will continue to expand into space until it fully dissipates and becomes part of the interstellar medium.  Sprinkled among the outward-fleeing debris are the newly forged atoms of heavy elements like gold, silver and calcium.

Dates Taken:
- 8/13/2007 through 9/17/2007

Exposures:
- This is a two-panel mosaic of the area
- 43x30-minute Ha sub exposures, about 21 hours in total, about 22 subs or 10.5 hours per panel

Equipment Used:
- TMB 203 F/7
- SBIG STL-6303
- Paramount ME
- Optec 3" rotator
- Starlight Instrument's Digital Feather Touch Focuser
- Astrodon 6nm Ha Filter

Processing:
CCDStack:
1)  Calibration with darks, flats, and bias frames
2)  Bloom rejection
3)  Impute (minor) bloomed pixels
4)  Image registration
5)  Normalization (Auto)
6)  Data rejection (Poisson sigma)
7)  Mean combine
8)  Hot/Cold Pixel rejection
9)  Impute hot/cold pixels
10) Deconvolution, Positive Constraint, 25 iterations

Photoshop CS2:
1) Overall “contrast curve”
2) Noise removal (NeatImage)
3) Noel Carboni’s Photoshop action for “local contrast enhancement”
4) Shadow-highlight to bring up the background data
5) Sharpening and additional deconvolution