New proper motions measured for 51 planetary nebulae were used in combination with previously available motions to calculate statistical parallaxes. The distance scale thus calibrated for optically thin nebulae agrees with that of Seaton (1966). For optically thick nebulae, the mean nebular absolute magnitude is -2.0. It is shown that nebulae of brightness absolute magnitude are not those of highest surface brightness. This violates a basic assumption of Seaton (1968), and thus invalidates his recalibration of the distance scale. The nebular absolute magnitudes are most consistent with a nebular evolutionary track that loops upward with the optically thick to thin transition occurring before the nebula reaches the peak of the track.
No takes yet. Share an insight, caveat, or question.
K. M. Cudworth (1974) studied this question.