SH2-173 is an emission nebula from the second edition of the Sharpless catalogue, a list of 313 nebulae published by Stewart Sharpless in 1959. Sharpless was a member of the staff at the US Naval Observatory in Flagstaff, where he surveyed hydrogen emission regions imaged in the Palomar Sky Survey. The mask-like appearance of SH2-173 led amateur astronomers to dub it the "Phantom of the Opera Nebula."
Gravitationally bound open clusters such as the Double Cluster indicate that multiple clusters may form within a single cloud of gas. SH2-173 itself lies within the rim of a large bubble of hydrogen gas in the Perseus arm of the galaxy, and contains several hot young stars that ionize nearby hydrogen and heat the surrounding dust. As this heated material expands, it creates shock waves and clumps in the cloud such as the dense region forming the "brow" of the Phantom's mask. The presence of numerous Young Stellar Objects (YSOs) in this area is an indicator of hierarchical cluster formation, as pressure from one cluster spawns another generation. For more detail see "Unveiling the birth and evolution of the H II region Sh2-173," Cichowolski et al. [
https://ui.adsabs.harvard.edu/abs/2009MNRAS.394..900C/abstract ]
The Phantom is quite faint...in an RGB image constructed from 40x60 seconds R, and 40x30 seconds each GB, I could see just a hint of reddish nebulosity. This image overlays the RGB stars with 365 minutes of Ha (73x300 seconds), presenting the nebula in brilliant red.
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