  By introducing a simplified transport model of outer layers of white dwarfs
we derive an analytical semi-empirical relation which constrains effective
temperature-mass-radius for white dwarfs. This relation is used to classify
recent data of white dwarfs according to their time evolution in non-accretion
process of cooling. This formula permit us to study the population map of white
dwarfs in the central temperature and mass plane, and discuss the relation with
the ignition temperature for C-O material. Our effective
temperature-mass-radius relation provide a quick method to estimate the mass of
newly observed white dwarfs from their spectral measurements of effective
temperature and superficial gravity.
