shell

Shell is a general term, one common use indicating a volume of one kind of interstellar medium (ISM) that is surround by another, generally meaning the inner ISM volume is hot and expanding, in which case the word bubble is also used. This describes volumes around OB stars (e.g., Strömgren spheres) and OB associations, and also describes supernova remnants. A supershell is an especially large shell, e.g., hundreds of parsecs across, often large enough to protrude out of the galactic disk. An HI supershell is a supershell evident from HI regions surrounding it. A shell's exact characteristics depend upon the progenitor(s), as well as the shell's age and history, and what type of ISM surrounds it. Shells are often spherically shaped or sometimes appear to be joined and/or curtailed spheres. The material interior to the shell can be extremely hot (e.g., 1,000,000 K) but the temperature presumably falls over time as it cools and expands. The expanding exterior can gather cold gas, but also can produce high temperatures from shock. Shells affect the pattern of nearby star formation: they can trigger it as well as damp it, triggering by pushing together cold gas and increasing its density, and/or damping it by pushing and carrying gas out of the galactic disk.

The solar system and neighboring stars are within a shell, the volume it surrounds known as the Local Bubble: it is a volume of ISM less dense and hotter than the surrounding ISM. Another example of a shell is the Anticenter Shell.

The term shell has other uses in astrophysics and planetary science, among them: electron shells, and regarding stellar structure, both the mass shells used in mathematical models, and in describing interior layers, such as those of early stars.