Abell 21 - The "Medusa Nebula"

Huge Planetary Nebula in Gemini

Uploaded 2/28/06

Select an image size for a larger view: 800 x 600 1200 x 800 1600 x 1200

This enigmatic object defies what we normally associate with the term "Planetary Nebula" in that it is so dim and ill defined, it seems more like an emission nebula instead. But the deep blue 16th magnitude central star defines its origin, and deep exposures such as this reveal its entire envelope. Normally captured as a dim arc in most images, my goals here were to expand upon the faint outer extensions first suggested to me by Ken Crawfords recent deep image.

Abell 21 in Gemini is a whopping 12 arcminutes across, and is rated as 11.3 magnitude. But its light is spread very thin, and most is concentrated in the right side of the nebula. Deeper hydrogen alpha images show a series of waves or ripples cutting across its face seen here well on the left side. In addition, a rippling patch of nebulosity on the left suggests an earlier outburst on the less denser, and less restrictive side of the object. Also, part of a small dim red cloud lies on the bottom left corner of this frame. A slight greenish cast can be seen around the central "hole" in the nebula. This ancient object does not have the energy to produce such colors in abundance, leaving a mostly reddish envelope.

There are several very faint galaxies in this frame, one very yellow distant one above the central star within the nebula, and a somewhat brighter 16th magnitude object to the lower right.

Processing: A standard G2V RGB image was taken, binned 1x1. Then an hour of supplemental H-alpha image set was secured. After creating a standard decent looking RGB image, and a strong, but low noise H-alpha representation, the images were combined tastefully by splitting off the R channel and mixing it 80 percent with the H-alpha data. Then the resulting image was combined again with the R channel using lighten to restore all of the stars. The resulting image was merged with the Green and Blue images to form this tri-color image. But we were not done yet. Because the H-alpha dominated the image even over the core, it was layered over the original RGB image with a layer mask to preserve the main arc and core regions, and only add H-alpha data on the periphery and outer zones.

Instrument: 12.5" f/5 Home made Newtonian Platform: Astrophysics 1200 QMD CCD Camera: SBIG 10XME NABG with Enhanced Water Cooling Guider: SBIG ST4 Exposure: Ha+RGB = 60:20:20:20 RGB Combine Ratio: 1: 1.05: 1.11 Filters: AstroDon RGB Tricolor Location: Payson, Arizona Elevation: 5150 ft. Sky: Seeing FWHM = 4 arcsec (Maxim DL - 10min subframe), Transparency 9/10 Outside Temperature: 30 F CCD Temperature: -30 C Processing Tools: Maxim DL, Gralaks Sigma, Photoshop, PixInsight, Starizona Debloomer. HOME GALAXIES EMISSION NEBS REFLECTION NEBS COMETS GLOBULARS OPEN CLUST PLANETARIES LINKS


FastCounter by bCentral