Abell 39
Abell 39
Planetary Nebula in Hercules

Click here for higher-resolution versions:  100% (2886x2886)  65% (1876x1876) 100% uncropped (4021x4046)
Click on image to toggle between a true-color version and a bi-color version


Abell 39 is a very faint planetary nebula, unusual for being quite spherical in shape. A "planetary nebula" (so called because the astronomer who first identified them as nebulae noted the color was similar to the then recently-discovered Neptune) is a structure of gas resulting from the death throes of a star about the size of our sun, when it runs out of fusable material; the color is the result of the gas being ionized by the remnant of the star, a white dwarf (ionized oxygen is the dominant emission in this planetary nebulae, giving off the characteristic blue-green color). The progenitor star is the blueish star at the center of the sphere, bluish because the white dwarf is so hot it emits UV radiation which then ionizes the surrounding oxygen.

Abell 39 is about 7,000 light years from us, and about five light years in diameter. The thickness of the "shell" is about 1/3 of a light year. The shell appears denser at the edges than in the middle as a result of simple geometry--we are looking through a thicker piece of it when viewed at the edge than at the center. Oddly enough, the progenitor star is not quite at the center of the nebula; it is not known why.

As with many photos of the night sky, the uncropped version of this shows many, many background galaxies; at least two are visible through the shell of the nebula (the oblong "stars" are galaxies, as are the round(ish) fuzzy things that don't look like stars). I cropped it for this presentation; I included an uncropped version in case anyone wants to explore the full "canvas."

I have presented this object in two different formats (each image is labeled in the lower left corner); each is interesting in its own way. This is the order in which they appear as you cycle through (by repeatedly clicking on the photo, waiting for each to download):

(i) A true-color version (the top photo in the stack), with the color created by imaging through red, green and blue filters (with a significant amount of Ha and OIII data blended into various channels, in varying percentages; Ha emissions are in the red spectrum, and OIII emissions are blue-green, so I have blended into the luminance layer and the red channel, and OIII into the green and blue channels, as well as the luminance layer. This one is a little more blue than the other.l

(ii) A version whose color is created by using the Ha data as the red channel, the OIII as the green channel and the OIII as the blue channel (OIII emissiona are blue/green); to preserve star colors, I used the RGB stars in this version.


Technical Information:

Ha:OIII:L:R:G:B: 480:510:510:180:180:240 (a total of 35 hours of light-frame exposure time); red and green exposures were all 15-minute exposures; blue all 20-minute exposures; Ha, SII and OIII were all 30-minute exposures.

Equipment: RC Optical Systems 14.5 inch Ritchey-Chretien carbon fiber truss telescope, with ion-milled optics and RCOS field flattener, at about f/9, and an SBIG STX-16803 camera with internal filter wheel (SBIG filter set), guided by an SBIG AO-X, all riding on a Bisque Paramount ME German Equatorial Mount.

Image Acquisition/Camera Control: Maxim DL, controlled with ACP Expert/Scheduler, working in concert with TheSky X Professional Edition.

Processing: All images calibrated (darks, bias and sky flats), aligned, combined and cropped in Pixinsight. Color combine in Pixinsight. Some finish work (background neutralization, color calibration) done in Pixinsight; LRGB image constructed in Photoshop, along with some finish work.

Location: Data acquired remotely from Sierra Remote Observatories, Auberry, California, USA.

Date: Images taken on many nights in May and June of 2021. Image posted June 29, 2021.

Date: Image scale of full-resolution image: 0.559 arcseconds per pixel.

Seeing: Generally excellent; individual calibrated luminance images varied from 1.6 arcsecond FWHM to 1.9 arcsecond FWHMr

CCD Chip temperature: -25C

Copyright 2021 Mark de Regt

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