This very dim emission nebula is located in the
constellation Cassiopeia. In images which do not
show quite so much of the dim clouds surrounding
Sharpless 173, some have seen a resemblance to the
"Phantom of the Opera" mask in the central portion.
In this deep image, it looks more like a puppy's face to me.

This image was published in "Astronomy" magazine,
April 2018, page 72.

Image data:
Telescope: Takahashi FSQ-106
Camera: SBIG STXL-11002
Exposure:
RGB - 90 minutes each
Hydrogen alpha - 25 hours
O-III - 3.5 hours (Look for the tiny green planetary nebula)
total exposure time = 33 hours