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.
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 () 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 ().
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 () come to a distance of 712 pc (2322 light years).
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.
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'.