An analysis of the variable radial velocity of alpha Cygni

On the basis of 447 radial velocities obtained at the Lick Observatory in the years 1927-1935, an attempt is made to discover the nature of the semiregular variability of alpha Cygni. Harmonic analysis of the 144 velocities obtained in 1931 suggests that this variability is due to the simultaneous excitation of many discrete pulsation modes. The amplitudes and periods of these modes are then determined by least-squares obtaining a solution that comprises 16 terms. All terms are found to have highly significant amplitudes, and most terms also pass a test of the stability of their amplitudes and phases. Reasons are given for believing that most terms represent nonradial oscillations, leading to the suggestion that the resulting surface motions are to be identified with macroturbulence. An argument is given for believing that the pulsational instability persists down to periods at which atmospheric oscillations become progressive, possibly giving rise to the observed mass loss.