P Cygni line profile
A feature in a spectrum that indicates a strong outflow of matter from a star. It is named after P Cygni, a Be star whose spectrum shows strong hydrogen and helium emission lines with absorption lines on their blueward side. These dual characteristics are caused by an expanding shell or wind of material that is being blown off the star either by radiation pressure or rapid rotation. Where the expanding shell is directly between us and the star, it produces absorption at a velocity which is blueshifted relative to the star. The other parts of the shell produce the emission lines. Conversely, if material is falling on to the star, the absorption component can be redshifted relative to the emission lines, causing an inverse P Cygni line profile.