SUNSET: 
0
Photographer:
Location of Photo:
Pommier Observatory, Portland, OR, USA
Date/Time of photo:
2017-09-23 through 2017-10-02
Equipment:
Celestron Compustar C14 with Starizona LF reducer/corrector (f/7,5). SBIG STL 11000 CCD camera with Baader Planetarium HaRGB filters. Exposures: HaRGB=210:190:190:190 minutes=13 hours total exposure.
Description:
The Cave Nebula (Sh2-155) lies at distance of 2,400 light-years in the constellation Cepheus. It is a large star-forming cloud of molecular hydrogen spanning approximately 10 light-years. Hot class O and B blue supergiant stars to the upper right of the image are ionizing the cloud's hydrogen gas, causing it to glow at the hydrogen-alpha wavelength of 656.28 nanometers. As their intense light evaporates gas from the cloud surface, it becomes ionized and the fierce stellar winds emitted by those stars blow it back against the cloud, forming a glowing, compressed rim. This pressure also triggers formation of more stars within the cloud. Residual dust within the cloud reflects and scatters the star light, causing portions to appear blue, just as air molecules in our atmosphere scatter sunlight causing a blue sky. This is one of the rare areas in the sky in which all three types of nebulae, red emission, blue reflection, and dark, can be seen within a small area.
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