Normalised emissivity (units are erg s⁻¹ cm³) of the Lyα line in a cloud of primordial gas at z = 4.8 as a result of recombination and collisional excitation processes as a function of temperature. There are three values of density, corresponding to overdensities of 1, 10, and 100, respectively; the mean cosmological hydrogen density corresponds to at this redshift. The dashed and dotted lines show the contribution from just recombination and collisional excitation, respectively.