Planetary nebulae do not always look like rings or blobs, and Sh2-188 is an example of that. It was photographed for the first time in 1965 and was initially thought to be a supernova remnant. However, the studies on the nebula composition revealed, that it is a planetary nebula – the abundance of nitrogen, oxygen, and carbon in the outer shell indicates that. When the medium-mass star ends its life, its core starts to compress due to gravity, and cooler outer layers expand and create the red giant star. During that phase, the star can lose up to 70%  of its total mass from its stellar wind.

When the core temperature reaches 100 million K, the available helium fuses into carbon and oxygen, and the core contraction stops for some time. The atmosphere gases continue to travel into interstellar space, and when the outer surface of the exposed core reaches about 30,000K, there is enough emitted ultraviolet light to ionize the ejected atmosphere, causing the gas to shine as a planetary nebula.

The asymmetric shape of Sh2-188 is explained by the interaction of the rapidly moving planetary nebula with the interstellar medium. As a result, the bow shock is preserved in the direction of forward motion, but the back end of the planetary nebula dissipates in space. Sh2-188 is considered to be one of the most extreme examples of planetary nebula and interstellar medium interaction.

Sh2-188 nebula in Cassiopeia
Sh2-188 nebula in Cassiopeia

Sh2-188 is also cataloged as Simeis 22, and the common names of this nebula are Dolphin or Shrimp. The diameter of the brightest arc is about 10 arc minutes, that is 1/3 of the apparent diameter of the Moon. But the nebula itself is quite faint, even its brightest part. And the weakest part at the back (with regards to the movement direction) is a challenge even for astrophotography. I captured the image above at my backyard observatory, under a suburban, light-polluted sky. Despite the significant exposures time and decent aperture, this weak part is almost invisible.

Clear skies!

Image technical data:

Date: October 2021
Location: Nieborowice, Poland
Telescope: Meade ACF 10" f/10
Corrector: AP CCDT67
Camera: QHY163M
Mount: SW EQ6
Guiding: GPCAM 224C + SW ED72
Exposure: RGB 40:30:40 minutes, Ha 3h, Oiii 1.5h
Conditions: Bortle 6, transparency good, seeing medium