SH2-185 (γ Cas Nebula, Ghost of Cassiopeia)

SH2-185, also known as the γ Cas nebula or the Ghost of Cassiopeia, is a mixed emission/reflection nebula in the constellation Cassiopeia. The nebula is ionized and illuminated by the star γ Cas and formed by its stellar wind. With a spectral type of B0.5IVe ('e' denotes an emission line star), γ Cas exhibits unusually high X-Ray emission for its class. The magnitude of the star varies slowly and is currently about 2.2. The distance of the star and the nebula is approximately 550 light-years away according to Hipparcos. The region also features some faint background nebulae, primarily visible in the infrared spectrum.

Click on the images for a full-scale/full-view version.

SH2-185 (gam Cas Nebula, Ghost of Cassiopeia) in H-alpha, blue and near infrared — This image is a false color composite where H-alpha (including red continuum) is mapped to red, blue is mapped to green and near infrared is mapped to blue. Stars are partially subtracted using continuum images.
The bright star is γ Cas. The reflection component of SH2-185 appears greenish (because it is bluish in reality), and the background nebulae, primarily visible in near infrared, appear blue.

SH2-185 (gam Cas Nebula, Ghost of Cassiopeia) in H-alpha, near infrared and blue — A detail in which green and blue are swapped, i.e. H-alpha is mapped to red, near infrared is mapped to green and blue is mapped to blue. Click on the image for the full view.
Compared to the image above, the infrared background nebulae become better visible. The small reflection near the right border is Ced 4a.

Both color composites suffer from artifacts caused by the bright star. In particular, these are:

H-alpha: Reflections between the image sensor and the filter. Due to the microlenses, the sensor surface acts like a reflective grating, resulting in periodic patterns of defocused stars as artifacts.
All colors: Halos caused by scattering in the atmosphere, possibly on very thin clouds of ice crystals. The effect is stronger for shorter wavelengths.
H-alpha: Internal filter reflections. This causes onion-like halos (rings whose brightness decreases step-wise with increasing radii).
All colors: A bias error caused by extreme sensor saturation in the pixel row of γ Cas. In the image, this results in a horizontal dark line going through the star.

Image data

FOV (full view in the JavaScript viewer):

1.33° × 0.89deg;

Date:

2022-2023

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:

H-alpha (3.5nm):	14.2h
Blue (SDSS B'):	7.0h
Near infrared (SDSS I'):	3.0h

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