The universe's cosmic catalog is filled with remarkable structures, including superbubbles—enormous cavities spanning hundreds of light-years, brimming with ultra-hot atomic gases heated to several hundred million degrees. These features emerge in the interstellar medium, sculpted by powerful stellar winds and supernova explosions.
Superbubbles are prevalent throughout the cosmos. Our Solar System resides near the center of one known as the Local Bubble, which stretches about 300 light-years across the interstellar medium in the Orion Arm. It encompasses the Local Interstellar Cloud and the G Cloud.
The hydrogen atom density there measures just 0.05 atoms/cm3; this exceptionally low density results from ongoing supernova radiation that disperses the gas.
OB associations host the most massive stars (spectral types O and B), with masses ranging from 8 to 100 solar masses. These powerhouses drive intense stellar winds and typically conclude their lives in supernova blasts.
Over a star's lifetime, these winds unleash energy equivalent to 1044 J—comparable to a single supernova explosion. Such outflows create stellar wind bubbles that can extend tens of light-years.
Related: What are galactic superwinds?
Though stars in OB associations lack gravitational binding, their proximity allows stellar wind bubbles to merge, forming expansive superbubbles.
When these massive stars perish, supernova shock waves propel superbubbles outward over hundreds of light-years at speeds of several hundred km/s.
Stars in OB groups drift apart slowly (around 20 km/s) while rapidly consuming fuel. Consequently, their supernovae detonate within the developing superbubble. These explosions contribute no remnants but inject energy as acoustic waves into the scorching interior, fueling the cavity's expansion.
Interstellar gas, pushed aside by the superbubble, cools and forms a surrounding shell. These shells were first detected via hydrogen's 21 cm line, inspiring early theoretical models. Observations later confirmed them in X-rays, infrared, and visible light. Superbubbles can also merge into even larger supershells.
