Milky Way in Cassiopeia and Cepheus

On this page a 24°×35° wide-field view of the Milky Way in constellations Cassiopeia, Cepheus and the north-eastern part of Cygnus is presented in different color composites. This region is full of smaller emission nebulae. For many of them the ionization sources an thus the distance can be determined which provides an insight in the 3D structure of the milky way in that direction.

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. The third section contains a presentation of some discoveries. Image and instrument data can be found at the end of this page.

Full views

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

Milky Way in Cassiopeia and Cepheus in H-alpha, blue continuum and red continuum — This image is a false color composite where H-alpha (including red continuum) is mapped to red, blue continuum (including [OIII] and H-beta emissions) is mapped to green and red continuum (without H-alpha) is mapped to blue. Reflection nebulae appear green to blue, HII regions are red. Stars in the continuum channels are partially subtracted in order to make the faint nebulae visible.

Milky Way in Cassiopeia and Cepheus in H-alpha — This visualization is a pseudo color image which only uses the H-alpha data (including some red continuum). It shows much more details of the emission nebulae than the image above.

**Color composition:** After partial star subtraction the dynamic range was compressed using a non-linear hi-pass filter. That leads to a compression ratio r which is used to calculate the color as depicted in the legend. (The legend shows the compression c:=1-r). Blue regions least compressed while white regions are most compressed. The luminance is determined by the tonal curve corrected result of the dynamic range compression.

Legend for false color image of Milky Way in Cassiopeia and Cepheus in H-alpha — This visualization is a pseudo color image which only uses the H-alpha data (including some red continuum). It shows much more details of the emission nebulae than the image above.

**Color composition:** After partial star subtraction the dynamic range was compressed using a non-linear hi-pass filter. That leads to a compression ratio r which is used to calculate the color as depicted in the legend. (The legend shows the compression c:=1-r). Blue regions least compressed while white regions are most compressed. The luminance is determined by the tonal curve corrected result of the dynamic range compression.

Milky Way in Cassiopeia and Cepheus in RGB — An almost-true color image. Unlike to the other images the stars are not subtracted. This improves the visibility of dark nebulae that absorb the light from the stars behind.

Due to limited resolution of continuum channels the image is only presented at half resolution.

Selected details

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

SH2-131 (containing the Elephant Trunk Nebula and SH2-129 in H-alpha and continuum — The upper large nebula is SH2-131. It contains the famous globule called the "Elephant Trunk Nebula" and is ionized by the star cluster IC 1396 in the center. The distance of this cluster is about 3000 light years and it lies on the periphery ot the Cepheus Bubble, also see the observations section.
The lower large nebula is SH2-129 and is ionized by the bright star near the center. According to recent estimations its distance is about 2300 light years, see ([2]).

SH2-171 ans SH2-170 — The larger nebula is Sh2-171. It is ionized by a star cluster in the center which lies in a distance of about 3500 light years. The smaller bright nebula on the left side is SH2-170, also see this larger view.

Observations and discoveries

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 latest Gaia data and thus provides an insight in 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 which are capable to ionize gas) with the names Cep OB2, Cep OB3 and Cep OB4 (from west to east). Cep OB2 is dominated by a the Cepheus Bubble, a large ring structure visible in infrared light ([1]) in 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 about 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 seems 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.
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 mainly appear in south of the galactic plane because the northern part of the Perseus arm seems to be obscured by dark nebulae.
Group 3: objects in other distances. SH2-129, which seems to lie on the periphery of the Cepheus bubble, belongs to group 3 because latest photometric estimations ([2]) come to a distance of 712 pc (2322 light years).
- In in H-alpha and continuum
- In in H-alpha

The views above shows many nebulae that cannot be found in catalogs. (The JavaScript Viewer makes it easy identify objects using catalogs or SIMBAD and to define new objects.) Some (probably not all) of these unexplored nebulae have been collected in the list below. Click on the following links for a presentation.

Uncataloged HII rings and arcs. Some of these structures may be random, some may be projections of spherical shells (bubbles). Such structures are usually circular in the image because stereographic projection was used. The outlines of these structures are completed to circles. It may be helpful to toggle these plots on and off by pressing the '2' key in order to improve the visibility.
- IA1-IA4 in H-alpha
- IA1-IA4 in H-alpha and continuum
Objects that are not cataloged by now and objects the are related to them (the list does not contain objects that are only minor extensions of known structures):
- I1-I33 in H-alpha
- I1-I33 in H-alpha and continuum
HII arcs near Zeta Cassiopeiae, a star hot enough to ionize hydrogen. The outer arc can be more or less completed to a ring. There might be a bubble and Zeta Cassiopeiae is close its surface. In this case the arcs we see consist in ionized hydrogen collected by the expanding bubble. Because Zeta Cassiopeiae is very close (about 180 pc or 590 light years) that bubble would be quite near.
The mentioned structures are faint and thus best visible in the H-alpha version of the image. It may be helpful to toggle plots on and off by pressing the '2'.
- IA5 and I34 in H-alpha

Notes

In the JavaScript viewer the object outlines can be toggled on and off by pressing the '2' key. That can be helpful in order to make certain structures (e.g. rings) visible.
False color images containing H-alpha and continuum:
- H-alpha is mapped to red, blue continuum is mapped to green and red continuum (without H-alpha) is mapped to blue.
- Color of molecular clouds and reflection nebulae in the false color image is something between green (bluish in reality) and blue (reddish in reality).
- HII regions (ionized hydrogen) appear red to orange, depending on the amount of OIII (doubly ionized oxygen) which is detected by the blue continuum filter.
SIMBAD queries for certain objects types can be made easily in JavaScript Viewer by drawing a circle and pressing a shortcut key or via menu
A repository with the discoveries can also be found at GitHub

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: 22h48, 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, 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:

Bias correction, dark current subtraction, flatfield correction
Alignment and brightness calibration using stars from reference image
Stacking with masking unlikely values and background correction
Star subtraction
Denoising and deconvolution both components (stars and residual)
RGB-composition (same factor for stars and residual for the true color composite)
Dynamic range compression using non-linear high-pass filter
Tonal curve correction