  Betelgeuse is a well known bright red supergiant that shows semi-regular
variations with four approximate periods of 2200, 420, 230, and 185 days. While
the longest period was customarily regarded as LSP (long secondary period) of
unknown origin, we identify the ~2200-d period as the radial fundamental mode,
and the three shorter periods as the radial first, second, and third overtones.
From a linear nonadiabatic pulsation analysis including the
pulsation/convection coupling, we have found that these radial pulsation modes
are all excited in the envelope of a model in a late stage of the core-carbon
burning. Models with similar pulsation property have masses of 11~12 M_\odot
(19M_\odot at ZAMS) with luminosities (log L/L_\odot =5.27~5.28) and effective
temperatures (log T_{eff}\approx 3.53) that are consistent with the range of
the observational determinations. We also find that a synthetic light curve
obtained by adding the fundamental and the first-overtone mode is comparable
with the light curve of Betelgeuse up to the Great Dimming. We conclude that
Betelgeuse is likely in the late stage of core carbon burning, and a good
candidate for the next Galactic Type II supernova.
