A relatively small cloud of dust and gas in the interstellar medium shaped like a comet with a bright-rimmed head. Cometary globules are situated near young massive stars with a strong stellar wind. The wind ionizes gases on the side facing the O stars and sweep away the low-density gas toward the tail. Cometary globules are believed to be molecular cloud condensations, which are so dense that they are not disrupted when an H II region expands into the molecular cloud surrounding it. The Rosette nebula is a good example of an H II region which shows an abundance of cometary globules.