Milky Way in Cassiopeia and Cepheus

On this page a 24°×35° wide-field view of the Milky Way in the constellations Cassiopeia, Cepheus and the northeastern part of Cygnus is presented in different color composites. This region is full of prominent emission nebulae. For many of them, the ionization sources are known, allowing their distances to be determined.

All pictures below are downscaled versions. Full resolution images with more than 100 megapixels can be loaded with a Javascript viewer by clicking on the images in the first section. Selected details are shown in the second section. An attempt to classify the objects according to their distance can be found in section “observations”. Image and instrument data are presented at the end of this page.

Full views

Click on the images to load a full resolution version with more than 100 megapixels using a JavaScript viewer.

Selected details

Here are a few details that also can be seen using the JavaScript viewer.

Observations

The constellations Cassiopeia and Cepheus are full of small emission nebulae. For many of them, the ionization sources are known. This allows distance measurements using the latest Gaia data and thus, provides insight into the 3D structure of the Milky Way in that direction.

The objects for which distances are known are divided into three groups. The following list contains a brief description and links to presentations for each group.

• Group 1: Objects in a distance between 850 pc and 1100 pc (2800 light years to 3500 light years): Literature assigns these objects to associations of OB stars (hot stars capable of ionizing gas) named Cep OB2, Cep OB3, and Cep OB4 (from west to east).

  Cep OB2 is dominated by the Cepheus Bubble, a large ring structure visible in infrared light ([1]) at an average distance of about 880 pc (2870 light years) and with an apparent diameter of about 10°. The real diameter in the mentioned distance is approximately 150 pc (500 light-years). On the periphery of that bubble, there are many young stars which ionize emission nebulae ([2]). Cep OB3 lies east of Cep OB2 and appears to be separated from Cep OB4 by a dark nebula. The distances roughly increase from west (CEP OB2) ot east (Cep OB4). Objects of group 1 are located in the local Orion arm of the Milky Way.

  • In in H-alpha and continuum (with star reduction)
  • In H-alpha (with star reduction)
  • In RGB without star reduction in order to better visualize the dark nebula
• Group 2: Objects in a distance between 2900 pc and 3300 pc (9500 light-years to 10700 light-years). These nebulae lie in the Perseus Arm of the Milky Way. Literature assigns these objects to OB associations Cep OB1, Cas OB2, and Cas OB5 (from west to east). In the images, objects of Group 2 primarily appear south of the galactic plane because the northern part of the Perseus Arm seems to be obscured by dark nebulae.
  • In in H-alpha and continuum (with star reduction)
  • In H-alpha (with star reduction)
  • In RGB without star reduction in order to better visualize the dark nebula
• Group 3: objects in other distances. Group 3: Objects at other distances. SH2-129, which appears to lie on the periphery of the Cepheus bubble, belongs to group 3 because the latest photometric estimations ([2]) place it at a distance of 712 pc (2322 light years).
  • In in H-alpha and continuum
  • In in H-alpha

Image data

Images where captured with a camera array which is described on the instruments page.

Image data are:

Projection type:	Stereographic
Center position:	RA: 22h48m, DEC: 59°
Orientation:	Above: North is right JavaScript viewer: North is up
Scale:	10 arcsec/pixel (in center at maximum resolution)
FOV:	34°×24° (RA×DEC, through center)
Exposure times:	Sum of exposure times of all frames used to calculate the image. H-alpha: 9.2 d Continuum channels: 6.1 d

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

References

1. Cepheus bubble as seen by the AKARI satellite in far infrared (50µm to 180µm)
2. Szilágyi, Máté & Kun, Mária & Ábrahá, Péter & Marton, Gábor. (2023). "The Gaia view of the Cepheus OB2 association". Monthly Notices of the Royal Astronomical Society. stad027.