  Red supergiant stars represent a late stage of the evolution of stars more
massive than about nine solar masses, in which they develop complex,
multi-component atmospheres. Bright spots have been detected in the atmosphere
of red supergiants using interferometric imaging. Above the photosphere of a
red supergiant, the molecular outer atmosphere extends up to about two stellar
radii. Furthermore, the hot chromosphere (5,000 to 8,000 kelvin) and cool gas
(less than 3,500 kelvin) of a red supergiant coexist at about three stellar
radii. The dynamics of such complex atmospheres has been probed by ultraviolet
and optical spectroscopy. The most direct approach, however, is to measure the
velocity of gas at each position over the image of stars as in observations of
the Sun. Here we report the mapping of the velocity field over the surface and
atmosphere of the nearby red supergiant Antares. The two-dimensional velocity
field map obtained from our near-infrared spectro-interferometric imaging
reveals vigorous upwelling and downdrafting motions of several huge gas clumps
at velocities ranging from about -20 to +20 kilometres per second in the
atmosphere, which extends out to about 1.7 stellar radii. Convection alone
cannot explain the observed turbulent motions and atmospheric extension,
suggesting that an unidentified process is operating in the extended
atmosphere.
