This is the second additional detail image spun off by
the Gamma Cassiopeia sky-skape, after the remote
Sh2-180 region.
Here we have the so-called Phantom of the Opera (actually a friendly retired galactic sailor) watching over Cassiopeia with his gruff but benevolent gaze. In Ha (revision B), his white long beard is more visible.
Technically speaking, some authors combine Sh2-172, Sh2-177 (both out of the field) and our Sh2-173 into one single star formation region. All three nebulae lie in the direction of the expanding shell of gas and dust surrounding the Cas OB5 association. Their estimated distance is 9000 ly.
The popularity of this object is a confirmation of pareidolia, that is the tendency of our brain to recognize objects, specially faces, where there are none. It makes the pair, IMHO, with our more dangerous tendency to pretend a causation effect, that leads to scapegoat and other cognitive mistakes. In our case, pareidolia is innocent and harmless, and we can enjoy it.
The word derives from the Greek words pará (παρά, "beside, alongside, instead of") and the noun eídōlon (εἴδωλον, "image, form, shape").
A second interpretation emerged in my family while discussing this image. In addition to the human face, one can see the head of a dog.
Figure: two different pareidolic interpretations of Sh2-173: a space man or a space dog?
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