Sh2-157 is an emission nebula, located in the Perseus Arm, one of the main spiral arms of the Milky Way, where the main OB associations visible in the constellation of Cassiopeia are also located, such as Cas OB5 and Cas OB2.In particular, this nebula is observed in the direction of Cas OB2. The nebula is associated with a large number of young stars, as is typical of the H II regions, including the brilliant WR 57, a Wolf-Rayet star with an estimated age of 7.5 million years, located inside of the cluster Mark 50 (see mouseover), located in the northern part of the cloud; this cluster is composed of some very large stars very close to each other, among which stands out HD 219460, a supergiant of the Wolf-Rayet class also cataloged as WR 157, which is a binary star whose companion is a bright giant of spectral class B1II with an orbital period of about two days. The evolutionary models of the region suggest that the star formation processes through this region of the Perseus Arm first affected the region from which the Cas OB2 association originated and which then extended to the formation of the cluster NGC 7510 and finally to Mark 50, which is therefore the product of the region's latest generation of stars. The nebula contains some very compact regions, among which the most conspicuous is Sh2-157A; this is a clearly visible concentration in the Hα band, where the main source of ionization of the gases seems to be a star of class O8 or B0. The cloud can be divided into two components: Component A, corresponding to the optically visible cloud, and Component B, which instead consists of the dark region detectable in the photographs. The amount of ionized hydrogen present in Component A is considerably less than the mass of the central star, which suggests that a certain amount of neutral hydrogen is also present; Component B is instead composed of unlit dust. Its distance is comparable with that of the diffuse nebula behind it, to which it belongs. A second, less bright agglomeration was classified as Sh2-157B. In addition to these structures is added an infrared radiation source known as IRAS 23138 + 5945, placed in association with the open cluster BDS2003. The two objects are about 2500 parsecs (8150 light years) from the Solar System.