SH2-274 (Medusa Nebula, Abell 21)

SH2-274, also known as the Medusa Nebula or Abell 21, is located in the constellation Gemini. Its progenitor is the white dwarf WD 0726+133, about 1900 light-years (590 pc) away. Including the faint northwestern extension (upper left in the images below), the nebula has an apparent diameter of about 0.43°, corresponding to a true diameter of roughly 14 light-years (4.5 pc).

SH2-274 (Medusa Nebula, Abell 21)
Click on the image to load it at full resolution in a JavaScript viewer.

In this false-color composite, [OIII] is mapped to red, Hα to green, and [SII] to blue. Stars are partially subtracted so that they appear white on average and to improve the visibility of the nebula. The progenitor, WD 0726+133, lies near the center of the bright part of the nebula.

SH2-274 (Medusa Nebula, Abell 21)
Compared to the image above, the mappings for red and blue are swapped, improving the visibility of [SII]. Specifically, [SII] is mapped to red, Hα to green, and [OIII] to blue. As above, stars are partially subtracted.

Image data

FOV (full view in the JavaScript viewer): 0.59° × 0.49°
Position (J2000): RA: 7h28m49s; DEC: 13°21′
Date: 2020-2024
Location: Pulsnitz, Germany
Instrument: 400mm Newton at f=1520mm
Camera Sensor: Panasonic MN34230
Orientation: North is up
Scale: 0.8 arcsec/pixel (at full resolution)
Total exposure times:
Hα (3 nm): 8.1 h
[OIII] (3 nm): 10.6 h
[SII] (3 nm): 14.2 h
Near infrared: 1.2 h
Blue: 1.2 h

Image processing

All image processing steps are deterministic and none of the algorithms use machine learning (often referred to as “AI”), which tends to generate plausible looking fake details. The software used can be downloaded here.

The image processing steps were:

  1. Bias correction, dark current subtraction, flatfield correction, noise estimation
  2. Alignment and brightness calibration using stars from reference image
  3. Stacking with outlier rejection, background estimation and optimal weighting based on noise estimation
  4. Star subtraction where star positions and intensities are extracted from continuum images
  5. Denoising and deconvolution of both components (stars and residual)
  6. Dynamic range compression using non-linear high-pass filter
  7. Color composition and tonal curve correction

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