2001ApJ...555..405D

The distribution of dust emitted by the supergiant star NML Cygni has been resolved by interferometry at 11 µm wavelengths at various times over a period of 6 yr.
Result. show there are two discrete dust shells, which have both moved away from the star approximately the same amount during the 6 yr period. This allows determination of the time between ejection of material forming the two shells to be 65±14 yr. Assuming the radial outflow velocity can be derived from Doppler-measured velocities of masers surrounding the star, its distance can be calculated from the observed angular motion to be 1220±300 pc. This decreases the luminosity of the star by about 1 mag over that deduced from the distance 1900 pc previously assumed.