Since the Gaia data release 2 , several works were published describing a bifurcation in the observed white dwarf colour - magnitude diagram for \mbox { $G _ { \mathrm { BP } } $ } { } - \mbox { $G _ { \mathrm { RP } } $ } { } > 0 . Some possible explanations in the literature include the existence of a double population with different initial mass function or two distinct populations , one formed by hydrogen - and one formed by helium - envelope white dwarfs . We propose instead spectral evolution to explain the bifurcation . From a population synthesis approach , we find that the spectral evolution occurs for effective temperature below { \simeq } 11 000 \mathrm { K } and masses mainly between 0.64 \mathrm { M } _ { \odot } and 0.74 \mathrm { M } _ { \odot } , which correspond to around 16 per cent of all DA white dwarfs . We also find the Gaia white dwarf colour-magnitude diagram indicates a star formation history that decreases abruptly for objects younger than 1.4 \mathrm { Gyr } and a top-heavy initial mass function for the white dwarf progenitors .