P Cygni

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P Cygni is a variable star in the constellation Cygnus.

Until the 17th century it was unknown, but then suddenly brightened to 3rd magnitude. It briefly disappeared and returned, and today has a magnitude of 4.82.

P Cygni is 5000 to 6000 light years from Earth, a hypergiant of spectral type B2 Ia. It is difficult to determine its exact distance, since the parallax is very small.

P Cygni gives its name to a type of spectroscopic feature called a P Cygni profile, where the presence of both absorption and emission in the spectral line profile indicate the existence of a gaseous envelope expanding away from the star. The emission lobe is redshifted and the absorption lobe is blueshifted with respect to the spectral line's rest wavelength. These profiles are useful in the study of stellar winds in many types of stars.

[edit] Coordinates (equinox 2000,0)

• Right ascension: 20h17m47.20s
• Declination: +38°01'59.0"