  Most aspects of stellar evolution proceed far too slowly to be directly
observable in a single star on human timescales. The thermally pulsing
asymptotic giant branch is one exception. The combination of state-of-the-art
modelling techniques with data assimilated from observations collected by
amateur astronomers over many decades provide, for the first time, the
opportunity to identify a star occupying precisely this evolutionary stage. In
this study, we show that the rapid pulsation period change and associated
reduction in radius in the bright, northern variable star T Ursae Minoris are
caused by the recent onset of a thermal pulse. We demonstrate that T UMi
transitioned into a double-mode pulsation state, and we exploit its
asteroseismic features to constrain its fundamental stellar parameters. We use
evolutionary models from MESA and linear pulsation models from GYRE to track
simultaneously the structural and oscillatory evolution of models with varying
mass. We apply a sophisticated iterative sampling scheme to achieve time
resolution $\le10$ years at the onset of the relevant thermal pulses.
  We report initial mass of $2.0\pm0.15\, \mathrm{M}_\odot$ and an age of $1.17
\pm 0.21$ Gyr for T UMi. This is the most precise mass and age determination
for a single asymptotic giant branch star ever obtained. The ultimate test of
our models will be the continued observation of its evolution in real time: we
predict that the pulsation periods in T UMi will continue shortening for a few
decades before they rebound and begin to lengthen again, as the star expands in
radius.
