This image is not my usual presentation style. I have adopted a slightly different palette for the nebula and, quite obviously, I have removed all the stars except for one that is located in the center of the image.

The star sports 45 identifiers in Simbad, but we'll call it by its HD Catalog name: HD 217086 (Since the image isn't plate-solved, the optical coordinates of the star are: 22 56 47.1887590800 +62 43 37.657173900). 

HD 217086 is a member of the Cepheus OB 3 Stellar Association and sits amidst a large, diffuse emission nebula called Sharpless 2-155, located in Cepheus. Informally called the "Cave Nebula", S155 is part of the larger Cepheus B Molecular Cloud, which includes a number of other stellar associations that I have previously imaged and reported on here on AB.

HD 217086 is the principal ionizing star of this nebula  responsible for the image you see here. Its UV radiation and winds are ionizing the surrounding hydrogen and are carving out huge cavities in the molecular cloud, creating supersonic shock waves that compress against other gases allowing other stars to form. It is an amazing process!

While informal and older sources cite a number of ionizing stars for the nebula, HD 217086 is the only proximal Type O star and thus the leading source of ionizing radiation for the region. There are a handful (half dozen) early Type B (B and B0) stars flung throughout the area that marginally contribute to the ionization, but all other stars are minor players compared to HD 217086.

HD 217086 is still classed as a "Young Stellar Object", which means it hasn't reached its main sequence stage. However, all the studies of the star indicate that it has arrived. It's specific spectral designation is:  O7Vnn((f))z C 

Type O, 7 = the cooler end of Type O spectrum, V = Main Sequence size, nn = broad absorption features, ((f)) show strong He II absorption and weak NIII emissions, z indicates a strong ionized Helium spectral line, and the C tells us there are C Carbon lines as well.

Quite the star. Type O stars are extremely rare, constituting a fraction of a fraction of a percent of all stars. They also live extremely short lives, measured in the millions of years. For HD 217086, life has just begun.

While we can see the obvious effect that HD 217086 has had on the surrounding molecular cloud, it is the radio and near IR studies that show S155 to be an extremely active star-formation region, most of it hidden from our view behind the backlit shadows in this image. 

I find the common name "Cave Nebula" to be so misleading. As always, these common names tend to put ideas into our heads about what we're supposed to see (a cave in this case) that only leads to serious confirmation bias. I think, though, that showing only the position of the main ionizing source, allows you to readjust your view of the region to see the effects of ionization as it spreads outward from the star. So, sit back and take a moment or two to reimagine what it is that is happening here in Cepheus.

Thanks for stopping by and taking a look. I hope you enjoyed the image and the reading.

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