M33 (Triangulum Galaxy)

M33 (also known as the Triangulum Galaxy) is a spiral galaxy in the constellation Triangulum, located about 2.73 million light-years from Earth. At this distance, its apparent diameter of 1.3° corresponds to a true diameter of about 62,000 light-years. M33 is the third-largest member of the Local Group, after M31 (the Andromeda Galaxy) and the Milky Way.

M33 (Triangulum Galaxy) in H-alpha and continuum
(requires JavaScript, Hα: on)
Click on the image to load it at full resolution in a JavaScript viewer. Use the button to switch the color mapping.

In both images, NIR (near infrared) is mapped to red, orange is mapped to green, and cyan is mapped to blue. In the version with Hα, that emission is shown in red.

M33 is relatively rich in emission nebulae. By toggling between the images (click the button), it can be seen that these emission nebulae (red in the version with Hα) correlate with bluish regions. This is because many of them are HII regions containing young blue stars that are also responsible for the ionization. Many of these blue giants can even be resolved in the image. Furthermore, the HII regions also emit [OIII] and Hβ light, which is captured by the blue filter.

Several large HII regions have their own designations. In the JavaScript viewer, these objects can be displayed by pressing the key 3 or via the menu.

Image data

FOV: 1.33° × 1.55° (mosaic composed from two tiles)
Position (J2000): RA: 1h33m52s; DEC: 30°39′
Date: 2022-2024
Location: Pulsnitz, Germany
Instrument: 400mm Newton at f=1520mm
Camera Sensor: Sony IMX455
Orientation: North is up (exactly in the image center)
Scale: 1 arcsec/pixel (at full resolution)
Total exposure times:
Hα: 35.6 h
Near infrared (SDSS I'): 14.4 h
Red (SDSS R' + 400-650 nm band-pass): 11.7 h
Blue (SDSS B'): 9.8 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. Denoising and deconvolution
  5. Dynamic range compression using non-linear high-pass filter
  6. Color composition and tonal curve correction

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