What is Ultraviolet Radiation?
Ultraviolet radiation is electromagnetic radiation with a wavelength shorter than that of visible light (between 400 nm and 10 nm), but longer than X-rays. This wavelength range corresponds to energies between 3 eV and 124 eV. Ultraviolet light is invisible to humans, because it corresponds to frequencies slightly higher than what the human eye detects as violet. Wavelengths of ultraviolet light between about 10 and 120 nm are termed ‘ionizing’ (they carry enough energy to liberate an electron from an atom or molecule). Lucky for us, these highly energetic waves are absorbed by the Earth’s atmosphere (especially by nitrogen atoms).
The energy required to ionize a neutral hydrogen atom is ≥ 13.6 eV. Photons with this energy have wavelengths ≥ 912 Å or 912 x 10^(-10) m, corresponding to far-ultraviolet (UV) radiation. Stars with temperatures greater than about 3 x 10^4 K (corresponding to an O-type star) emit a large number of these energetic photons and can thus ionize a significant amount of the interstellar medium surrounding them. The photoionized gas surrounding one of these hot stars is termed an ‘HII Region.’ A particularly striking HII region (NGC 604) is seen in the Triangulum Galaxy (shown below).
In 1939, Bengt Strömgren considered the case of a fully ionized, spherical region of uniform density hydrogen gas in which the ionizing photons come from a central hot star. The radius of this so-called Strömgren sphere is determined by calculating the steady-state solution where ionization is balanced by hydrogen recombination:
If you are interested in learning more about HII Regions, you should check out this website maintained by NRAO:
Or, try reading Chapter 15 of Bruce Draine’s book Physics of the Interstellar and Intergalactic Medium. Finally, you might consider checking out the website create by Stephen Portillo for the same class on the ISM. This website shows how the physical properties at the photoionization boundary of an HII region can be determined from Hubble Space Telescope images in the case of the famous Pillars of Creation: