right|250px|thumb|the rosette nebula. the strÃ¶mgren sphere can be seen approximately in the middle of the image. in theoretical astrophysics, there can be a sphere of ionized hydrogen (h ii) around a young star of the spectral classes o or b. the theory was derived by bengt strÃ¶mgren in 1937 and later named strÃ¶mgren sphere after him. the rosette nebula is the most prominent example of this type of emission nebula from the h ii-regions. == the physics == very hot stars of the spectral class o or b emit very energetic radiation, especially ultraviolet radiation, which is able to ionize the neutral hydrogen (h i) of the surrounding interstellar medium, so that hydrogen atoms lose their single electrons. this state of hydrogen is called h ii. after a while, free electrons recombine with those hydrogen ions. energy is re-emitted, not as a single photon, but rather as a series of photons of lesser energy. the photons lose energy as they travel outward from the star's surface, and are not energetic enough to again contribute to ionization. otherwise, the entire interstellar medium would be ionized. a strÃ¶mgren sphere is the theoretical construct which describes the ionized regions. == the model == in its first and simplest form, derived by the danish astrophysicist bengt strÃ¶mgren in 1939, the model examines the effects of the electromagnetic radiation of a single star (or a tight cluster of similar stars) of a given surface temperature and luminosity on the surrounding interstellar medium of a given density. to simplify calculations, the interstellar medium is taken to be homogeneous and consisting entirely of hydrogen. the formula derived by strÃ¶mgren describes the relationship between the luminosity and temperature of the exciting star on the one hand, and the density of the surrounding hydrogen gas on the other. using it, the size of the idealized ionized region can be calculated as the strÃ¶mgren radius. strÃ¶mgren's model also shows that there is a very sharp cut-off of the degree of ionization at the edge of the strÃ¶mgren sphere. this is caused by the fact that the transition region between gas that is highly ionized and neutral hydrogen is very narrow, compared to the overall size of the strÃ¶mgren sphere. the above-mentioned relationships are as follows: :* the hotter and more luminous the exciting star, the larger the strÃ¶mgren sphere. :* the denser the surrounding hydrogen gas, the smaller the strÃ¶mgren sphere. in strÃ¶mgren's model, the sphere now named s
Astrophysics