Along with the brown bear, it is one of only two of the eight modern bear species not considered globally threatened with extinction by the IUCN. American black bears often mark trees using their teeth and claws as a form of communication with other bears, a behavior common to many species of bears.
Planetary nebulae are astronomical objects made up primarily of gaseous materials. They are extended in size and fuzzy in appearance, and generally show some degree of symmetry. The nebula is illuminated by a central star, which sometimes is too faint to be seen.
Planetary nebulae, some of the loveliest objects in the sky, are complex shells of gas that have been ejected by dying advanced giant stars like Mira. They were discovered as a set and given their collective name by William Herschel in the late 1700s. The story is given in the text for the first planetary, NGC 7009.