  Betelgeuse is an M supergiant with a complex and extended atmosphere, which
also harbors spots and giant granules at its surface. A possible magnetic field
could contribute to the mass loss and to the heating of the outer atmosphere.
We observed Betelgeuse, to directly study and infer the nature of its magnetic
field. We used the new-generation spectropolarimeter NARVAL and the least
square deconvolution (LSD) method to detect circular polarization within the
photospheric absorption lines of Betelgeuse. We have unambiguously detected a
weak Stokes V signal in the spectral lines of Betelgeuse, and measured the
related surface-averaged longitudinal magnetic field Bl at 6 different epochs
over one month. The detected longitudinal field is about one Gauss and is
apparently increasing on the time scale of our observations. This work presents
the first direct detection of the magnetic field of Betelgeuse. This magnetic
field may be associated to the giant convection cells that could enable a
"local dynamo:.
