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.
A diffuse nebula also may occur near a cool star. In this case, the ultraviolet light from the star is too weak to make the nebula's gas atoms give off light. But the dust particles in the diffuse nebula reflect the starlight.Astronomers refer to this kind of diffuse nebula as a reflection nebula.If a diffuse nebula occurs in an area that has no nearby stars, it neither emits nor reflects enough light to be visible. In fact, its dust particles blot out the light from the stars behind them. diffuse nebula also may occur near a cool star. In this case, the ultraviolet light from the star is too weak to make the nebula's gas atoms give off light. But the dust particles in the diffuse nebula reflect the starlight.