Sh2-173 is an emission nebula lying 8,800 light-years from Earth in the constellation Cassiopeia. It is part of the Perseus arm of our Milky Way Galaxy. Its shape and dark dust clouds bear a resemblance to the mask worn by the Phantom of the Opera in the Broadway musical or the film starring Lon Chaney. Hence it is commonly referred to as the Phantom of the Opera Nebula. It is extremely faint, requiring long exposures.