Fig9_3 Spheres of ionization

Fig9_3 Spheres of ionization

Fig. 9.3 . Ultraviolet radiation, denoted by uv, from a hot star ionizes hydrogen and other atoms in its immediate vicinity, creating a nebulous region that radiates emission lines and contains abundant ionized hydrogen, denoted H II. They are known as emission nebulae or H II regions. In this figure, the size of the atoms, ions, electrons and protons are greatly exaggerated in order to visualize them. The ionization by the uv creates numerous free electrons and protons that are not attached to atoms. They subsequently recombine to make atoms in a process of continued disruption and reconciliation. The free electrons can emit two kinds of radiation, illustrated in Figure 9.4. At large distances from the star, its ultraviolet rays are all absorbed, and can travel no further into surrounding space. This limits the radius of the emission nebula, or H II region, to the Strömgren radius at about 30 light-years.

Copyright 2010, Professor Kenneth R. Lang, Tufts University