P Cygni is a variable star in the constellation Cygnus. It is a hypergiant luminous blue variable (LBV) star of spectral type B1Ia+ that is one of the most luminous stars in the Milky Way galaxy. The star is located about 5000 to 6000 light years from Earth. It was unknown until the 17th century, when it suddenly brightened to 3rd magnitude. It briefly disappeared and returned, and today has a magnitude of 4.8 ± 0.5.

Luminous Blue Variable stars like P Cygni are very rare and short lived, and only form in regions of galaxies where intense star formation is happening. LBV stars are so massive and energetic (typically 50 times the mass of our sun and tens of thousands of times more luminous) that they exhaust their nuclear fuel very quickly. After shining for only a few million years (compared to several billion years for our sun) they erupt in a supernova. The recent supernova SNG2006gy was likely the end of an LBV star similar to P Cygni but located in a distant galaxy.

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. They are often cited as an indicator of a Luminous blue variable star.

References

• http://laserstars.org/spectra/PCygni.html
• http://www.peripatus.gen.nz/Astronomy/objPCyg.html