An overview

The classical spectroscopic signature of mass loss, first reviewed in the literature by Beals (1950), is the so-called P Cygni line profile. This label has come to be attached to the profile shape in which blueshifted absorption sits alongside redshifted emission. In truth, the practice of describing just this configuration as ‘P Cygni’ does little justice to the rich variety of profile forms that are to be found in this famous star's optical spectrum—Beals himself put the case for 4 different profile types characteristic of ‘P Cygni stars.’ Interestingly, from the perspective of this collection of papers on line emission, these other forgotten types include forms that emphasise emission rather than absorption. Indeed, those of us who have taken spectra of P Cygni itself are painfully aware of just how strong the strongest emission features (in Hα, He i λ5876) really are!