Inastronomy asuperbubble orsupershell is a cavity which is hundreds of light years across and is populated with hot (106K) gas atoms, less dense than the surroundinginterstellar medium, blown against that medium and carved out by multiplesupernovae andstellar winds. The winds, passage and gravity of newly born stars strip superbubbles of any other dust or gas.[2] TheSolar System lies near the center of an old superbubble, known as theLocal Bubble, whose boundaries can be traced by a sudden rise industextinction of exterior stars at distances greater than a few hundred light years.[citation needed]
The most massive stars, with masses ranging from eight to roughly one hundred solar masses andspectral types of O and early B, are usually found in groups called OB associations. Massive O stars have strong stellar winds, and most of these stars explode assupernovae at the end of their lives.
The strongest stellar winds release kinetic energy of 1051ergs (1044J) over the lifetime of a star, which is equivalent to a supernova explosion. These winds can formstellar wind bubbles dozens of light years across.[3]InsideOB associations, the stars are close enough that their wind bubbles merge, forming a giant bubble called a superbubble. When stars die, supernova explosions, similarly, driveblast waves that can reach even larger sizes, with expansion velocities up to several hundred km s−1. Stars in OB associations are not gravitationally bound, but they drift apart at small speeds (of around 20 km s−1), and they exhaust their fuel rapidly (after a few millions of years). As a result, most of their supernova explosions occur within the cavity formed by the stellar wind bubbles.[4][5] These explosions never form a visiblesupernova remnant, but instead expend their energy in the hot interior as sound waves. Both stellar winds and stellar explosions thus power the expansion of the superbubble in the interstellar 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 attwenty-one centimeters fromhydrogen,[6] leading to the formulation of the theory of superbubble formation. They are also observed inX-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 visible 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 themsupershells.
Large enough superbubbles can blow through the entire galactic disk, releasing their energy into the surrounding galactic halo or even into theintergalactic medium.[7][8]
^Tomisaka, K.;Ikeuchi, S.[in Japanese] (1986). "Evolution of superbubble driven by sequential supernova explosions in a plane-stratified gas distribution".Publ. Astron. Soc. Jpn.38 (5):697–715.Bibcode:1986PASJ...38..697T.
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