supershell

A supershell is a very large shell, i.e., a very large portion of the interstellar medium (ISM) that surrounds some volume, the shell having some material distinct from that within and that around it. The interior material is generally hot and expanding, enlarging the supershell. The word superbubble is also used, and both terms are sometimes meant to include the material within the cavity as well. An HI supershell is a supershell that is evident from HI regions at its borders (or comprising the surrounding shell). Supershells can be large enough to protrude out of the galactic disk. The interior gas may be contained behind the dense region it is pushing out, or may leak, and is considered a likely source of the thin plasma within galactic halo (beyond the disk) termed the galactic corona or gaseous corona. 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 push cold gas together, but also can increase temperatures in surrounding gas from shock. Shells affect the pattern of nearby star formation: pushing together cold gas can trigger it, but pushing cold gas out of the galactic disk can damp it.

Shells seem to be driven by some kind of energetic event, but supershells generally show so much excess energy that a single supernova would be insufficient, i.e., far more energy than a foe. Their formation is of research interest; within the supershell, sometimes there is an OB association and/or evidence of supernovae, lending evidence to such progenitors and in many cases hypothesized to be the combined shells of several such progenitors, which may result from a type of chain reaction, with early shells triggering star formation including OB stars that then form subsequent shells, including through supernovae. Another general theory is they can be formed by collisions of high-velocity clouds with the ISM of the galactic disk, and I believe some astrophysicists theorize both processes create them, likely producing observable differences. One model of galaxy star formation consists of volumes of gas "blown out" of the galactic disk, later to fall back in to cause more such events, being an underlying mechanism for a galaxy's periodic high and low star formation rates.