The cosmic catalog contains many objects, including superbubbles. These are cavities extending over several hundred light-years and filled with extremely hot atomic gases, heated to several hundred million degrees. They form in the interstellar medium, sculpted by stellar winds and supernova explosions.
Superbubbles are not uncommon in the Universe. The Solar System is located near the center of a superbubble, called Local Bubble, extending about 300 light-years in the interstellar medium of the Orion arm. It contains the Local Interstellar Cloud and the G Cloud.
The density of hydrogen atoms there is 0.05 atoms/cm 3 ; this very low density is due to the presence of a supernova continuing to emit powerful radiation, thus dissipating the gas.
The OB stellar groups contain the most massive stars (spectral type O and B) whose masses can vary between 8 and 100 solar masses. These stars are the source of strong stellar winds and usually end their life as a supernova.
During the life of such a star, the stellar winds release the equivalent of 1044 J of energy, or as much as the explosion of a supernova. These winds can form bubbles of stellar winds that can span tens of light-years.
Related:What are galactic superwinds?
Although stars in OB associations are not gravitationally bound, they are close enough that their stellar wind bubbles coalesce and then form a larger structure called a superbubble.
When massive stars die, the shock waves produced by supernovae can expand these superbubbles over several hundred light-years, with an expansion speed of several hundred km/s.
Even though stars in OB groups move away from each other, they do so at a relatively slow speed (about 20 km/s) while quickly depleting their fuel. So much so that, when they explode, their supernova necessarily takes place in the superbubble in formation. The explosions thus leave no supernova remnant but bring their energy into the extremely hot medium of the superbubble, in the form of acoustic waves, thus feeding the expansion of the cavity.
The interstellar gas, repelled by the superbubble, forms a shell around it as it cools. These gas shells were initially observed via the line 21 cm from hydrogen, making it possible to formulate the first theoretical model of superbubbles. They were then observed in X-rays, infrared and visible light. Superbubbles can also fuse, resulting in outer structures called supershells.
