Everything about Superbubble totally explained
Superbubble is the
astronomical term used to describe a cavity hundreds of light years across filled with 10
6 K gas blown into the
interstellar medium by multiple
supernovae and
stellar winds. Sol lies near the center of an old superbubble, whose boundaries can be traced by a sudden rise in
dust extinction of stars at distances greater than a few hundred light years.
The most massive stars, with masses ranging from eight to roughly one hundred solar masses and
spectral types of O and early B are usually found in groups called OB associations. Massive O stars have strong stellar winds, and all of these stars explode as
supernovae at the ends of their lives.
The strongest stellar winds release kinetic energy of 10
51 ergs (10
44 J), equivalent to a supernova explosion. These winds can blow
stellar wind bubbles dozens of light years across.
Supernova explosions, similarly, drive
blast waves that
can reach even larger sizes, with expansion velocities of as much as
several hundred km s
-1.
Stars in OB associations are not gravitationally bound, but only drift
apart at speeds of around 20 km s
-1. As a result,
most of their supernova explosions occur within the cavity carved by
the strongest stellar wind bubbles. They never form a visible
supernova remnant, but
instead efficiently deposit their energy into the hot interior as sound waves. Large enough superbubbles can blow entirely through the galactic disk, releasing their energy into the surrounding galactic halo or even into the
intergalactic medium.
The interstellar gas swept up by superbubbles generally cools, forming a dense shell around the cavity. These shells were first observed in line emission at twenty-one centimeters from
hydrogen, leading to the formulation of the theory of superbubble formation. They are also observed in
X-ray emission from their hot interiors, in optical line emission from their ionized shells, and in infrared continuum emission from dust swept up in their shells. X-ray and human optical emission are typically observed from younger superbubbles, while older, larger objects seen in twenty-one centimeters may even result from multiple superbubbles combining, and so are sometimes distinguished by calling them
supershells.
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