  Planetary Nebulae (PN) emit enormous amount of energy in several emission
lines. Measuring the line-flux for PNe in a given stellar population, the
Planetary Nebula Luminosity Function (PNLF) can be compiled. Surveys of PNe
revealed that the faint-end of the PNLF can be approximated by a simple
exponential dependency expected for an expanding spherical shell. However at
the bright-end there exists a steep cut-off which was unexpected and remains
unexplained. Interestingly, the cut-off value appears to be nearly the same for
different stellar populations as young spiral galaxies and old elliptical
galaxies and, despite the lack of understanding, became an extragalactic
distance estimator. Here we show that the recently computed post-AGB
evolutionary tracks are capable to explain the decades old mystery. All new
models with ages between 1 and 7 Gyr (progenitor masses between 2.0 and 1.1 of
solar mass) evolve fast enough to ionize the PN, and at similar post-AGB
luminosity which allows the [O III] 500.7nm line to reach nearly the same
magnitude. The new models predict that the Sun at the end of its life will form
a rather faint PN.
