M101 (Pinwheel Galaxy) in H-alpha and continuum light

M101 (also known as Pinwheel Galaxy) is a spiral galaxy in constellation Ursa Major which lies in a distance of about 21 million light-years. Its a large galaxy with a diameter of about 170,000 light-years and it is the largest member of the M101 group which belongs (together with many others including the Local Group) to the Virgo Supercluster.

Click on the image to open it in a JavaScript viewer.

M101 (Pinwheel Galaxy) in H-alpha and continuum light
(requires JavaScript, H-alpha: on)
In both images NIR (near infrared) is mapped to red, yellow is mapped to green and blue is mapped to blue. In the version with H-alpha that emission line is shown in reddish orange.

By toggling between the images (click on the button) it can be seen that the HII regions (reddish) correlate with bluish regions. That's because the HII gas clouds typically contain many young (blue) stars which are also responsible for the ionization. Furthermore the HII regions also emit [OIII] and H-beta light which is collected by the blue filter.

Many bright HII regions have their own NGC numbers. Press the '3' key in the JavaScript viewer or use the menu in order to plot their outlines.

Image data

FOV: 0.5° × 0.44° (full view)
Date: 2019-2022
Location: Pulsnitz, Germany
Instrument: 400mm Newton at f=1520mm
Camera Sensor: Panasonic MN34230
Orientation: North is up (approximately)
Scale: 0.8 arcsec/pixel
Total exposure times:
H-alpha (3nm): 20.3 h
NIR: 3.6 h
Yellow (540nm to 650nm): 6.3 h
Blue: 3.7 h

Image processing

All image processing steps are deterministic, i.e. there was no manual retouching or any other kind of non-reproducible adjustment. The software which was used can be downloaded here.

Image processing steps where:

  1. Bias correction, dark current subtraction, flatfield correction
  2. Alignment and brightness calibration using stars from reference image
  3. Stacking with masking unlikely values and background correction
  4. Denoising and deconvolution
  5. Color composition
  6. Dynamic range compression using non-linear high-pass filter
  7. Tonal curve correction

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