Résumé
In the solar limb, linear polarisation is due to anisotropy of the radiation
field induced by limb darkening. It is maximal when it is seen parallel to the
limb and it vanishes when it is integrated over the spherically-symmetric solar
disk. Therefore for distant stars, that present spherical symmetry, linear
polarisation signatures are very difficult to observe. However strong linear
polarisation features have been reported in the prototypical red supergiant
star alpha Ori (Betelgeuse). With an analytical model we propose to explain
them.