On the spectrum of a gaseous nebula of pure hydrogen

The Balmer decrement was calculated for a homogeneous, spherical, pure hydrogen nebula, ionised and excited both by radiative and collisional processes. Reabsorption in the Balmer series from sublevels 2s and 2p was taken into account. Calculations were carried out for electron temperatures of 10⁴ K and 2×10⁴ K, electron densities from 10³ to 10⁶ cm^–3, and radii from 10¹⁵ to 10¹⁷ cm. The results define allowed and forbidden regions in the log(H/H) vs log(H/H) diagram different from those of previous results obtained assuming purely radiative or purely collisional ionisation and excitation. A brief comparison with observations is presented and discussed.