P Cygni: An Extraordinary Luminous Blue Variable

P Cygni is a prototype for understanding mass loss from massive stars. This textbook star is known first of all because of two great eruptions in the 17th century. In the first half of this century it has given its name to a class of stars which are characterized by spectral lines consisting of nearly undisplaced emissions accompanied by a blue-displaced absorption component. This characteristic P Cygni-type profile betrays the presence of a stellar wind, but P Cygni's wind is quite unlike that of other hot supergiants. P Cygni was the first star that showed the effects of stellar evoluton from a study of its photometric history. It shares some common properties with the so-called Luminous Blue Variables. However, P Cygni is a unique object. This review deals with P Cygni's photometric properties, its circumstellar environment - including infrared and radio observations - and its optical and ultraviolet spectrum. Smaller sections deal with P Cygni's wind structure and evolution.