Final answer:
Open objects like open clusters, groups of O and B stars, and planetary nebulae are listed in the category of stellar associations, which include star clusters loosely bound by gravity.
Step-by-step explanation:
Open objects are listed in the stellar associations category because they are collections of stars loosely bound by gravitational forces. Specifically, an open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and are relatively close to each other. A group of O and B stars refers to young, hot, high-mass stars that emit large amounts of ultraviolet radiation and are often found in open clusters. Planetary nebulae, on the other hand, are the remnants of stars that were once similar to our Sun; after these stars have exhausted their nuclear fuel, their outer layers are expelled, leaving behind a hot core that illuminates the surrounding gas.