M51 (NGC 5194 and NGC 5195, Whirlpool Galaxy) in H-alpha and continuum light

M51 (also known as Whirlpool Galaxy) are two interacting galaxies in constellation Canes Venatici. The larger one (M51A or NGC 5194) is a spiral galaxy. Its smaller companion (M51B or NGC 5195) is a dwarf galaxy that is highly distorted from the interaction with M51A. The galaxy pair lies at a distance about 25 million light years. The diameter of the visible disk of larger galaxy is about 90,000 light years. The smaller dwarf galaxy is about half as large. M51A is the brightest member of the M51 group.

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M51 (NGC 5194 and NGC 5195, Whirlpool 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.

It seems that M51A contains many young (bluish stars) while M51B and its tidal streams mainly consists in older (brownish grey) stars (better visible in the version without H-alpha). Furthermore it can be seen that the dust band of the upper arm of M51A obfuscates M51B, i.e. obviously M51B lies behind M51A.

Image data

FOV: 0.44° × 0.44° (full view)
Date: 2020-2022
Location: Pulsnitz, Germany
Instrument: 400mm Newton at f=1520mm
Camera Sensor: Panasonic MN34230
Orientation: North is up (exactly)
Scale: 0.8 arcsec/pixel
Total exposure times:
H-alpha (3nm): 20.0 h
NIR: 3.7 h
Yellow (540nm to 650nm): 6.1 h
Blue: 3.9 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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