Sh2-290 (also known as Abell 31, PK 219.1+31.2, A 31 or ARO 135) is a planetary nebula in the constellation Cancer. It is very difficult to be detected about 2.5 degrees at SSE of the open cluster M67; despite its large size (one of the largest planetary nebulae of the sky) it is not as bright, indeed its extent is the cause of its low brightness: it is in fact a very ancient nebula, whose gases have expanded to such an extent that it begins to disperse in the surrounding interstellar medium. The southern part of the cloud is surrounded by a bow shock effect, while the northern part has extremely nuanced limits, a sign that dissolution is already taking place here. All planetary nebulae, over time, will suffer this same fate.