Strömgren sphere

Summary

A Strömgren sphere is the region of ionised hydrogen around a hot star in a uniform cloud of hydrogen gas. Its radius is set by a balance of two rates. One is the number of ionising photons the star emits per second; the other is the number of recombinations per second inside the sphere. Inside the sphere the gas is almost fully ionised, and the transition to neutral gas outside is sharp. It is the simplest model of an H II region such as the Orion Nebula.