P Cygni

An unusual variable star, the spectral lines of which have a particularly characteristic profile interpreted in terms of an expanding envelope around the star. P Cygni is a recurrent nova. It was recorded as third magnitude in August 1600 and stayed at this brightness for six years before fading slowly. A second outburst occurred in about 1655, which was again followed by slow fading. It subsequently fluctuated in brightness around sixth magnitude and has been about fifth magnitude, with only small variations, since 1715. The lines in the spectrum of P Cygni are all double, consisting of a broad emission line with a narrower absorption line adjacent on the blue side. The absorption comes from starlight passing through surrounding shells of material, while the emission comes from the portions of the shells either side of the central star as viewed from the Earth. The emission and absorption components are displaced from each other by the Doppler effect because the shells are expanding. Detailed analysis has shown that there are three distinct shells, the outermost of which is pulsating with a 114-day period. Similar line profiles are observed in the spectra of other objects surrounded by expanding envelopes and are described as P-Cygni profiles.