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 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 gather cold gas, but also can produce high temperatures from shock. Such shells affect patterns 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.

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.