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.
Cancel. A nebula is an interstellar cloud that is comprised of dust and light elements such as hydrogen and helium. Find out about ho gravity and pressure keep nebulas from collapsing with help from an applied physics professional in this free video clip.Part of the Video Series: Such Great Physics.ideo Transcript. Hello, my name is Walter Unglaub, and this is how do gravity and pressure keep nebulas from collapsing. A nebula is an interstellar cloud that is comprised of dust and light elements such as hydrogen and helium.