P Cygni's characteristic and eponymous line profile for H-α
Epoch J2000 Equinox J2000
|Right ascension||20h 17m 47.2018s|
|Declination||+38° 01′ 58.549″|
|Apparent magnitude (V)||4.795
3 to 6
|U−B color index||-0.58|
|B−V color index||+0.42|
|Variable type||Luminous Blue Variable|
|Radial velocity (Rv)||-8.9 km/s|
|Proper motion (μ)||RA: -3.53 mas/yr
Dec.: -6.88 mas/yr
|Parallax (π)||0.52 ± 0.50 mas|
|Absolute magnitude (MV)||−7.9|
|Rotation||75 km/s (50 days)|
NOVA Cyg 1600, * 34 Cyg, Glazar Cyg 11, JP11 3218, TD1 26474, GSC 03151-03442, LS II +37 50, TYC 3151-3442-1, AG+37° 1953, HBHA 3703-88, 2MASS J20174719+3801585, ALS 11097, HD 193237, MCW 849, BD+37° 3871, Hen 3-1871, PLX 4837, CEL 5017, PPM 84645, V* P Cyg, Hilt 916, RAFGL 5493S, GC 28218, HIP 100044, ROT 2959, GCRV 12673, HR7763, SAO 69773, AAVSO 2014+37A.
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 the Earth. It was unknown until the end of the 16th century, when it suddenly brightened to 3rd magnitude. It was first observed on 18 August (Gregorian) 1600 by Willem Janszoon Blaeu, a Dutch astronomer, mathematician and globe-maker. After six years the star faded slowly, dropping below naked-eye visibility in 1626. It brightened again in 1655, but had faded by 1662. Another outburst took place in 1665; this was followed by numerous fluctuations. Since 1715, however, P Cygni has been a fifth magnitude star, with only minor fluctuations in brightness. Today it has a magnitude of 4.8 ± 0.5.
Due to its erratic nature, P Cygni has sometimes been called a "permanent nova"; however, its behaviour is no longer thought to involve the same processes associated with true novae.
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 SN 2006gy  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. 
- T Tauri stars
- "P Cygni". SIMBAD Astronomical Database. Retrieved 2010-11-25.
- "GCVS Query=P Cyg". General Catalogue of Variable Stars @ Sternberg Astronomical Institute, Moscow, Russia. Retrieved 2010-11-25.
- Balan, A.; Tycner, C.; Zavala, R. T.; Benson, J. A.; Hutter, D. J.; Templeton, M. (2010). "The Spatially Resolved Hα-emitting Wind Structure of P Cygni". The Astronomical Journal 139 (6): 2269. arXiv:1004.0376. Bibcode:2010AJ....139.2269B. doi:10.1088/0004-6256/139/6/2269.
- Van Genderen, A. M. (2001). "S Doradus variables in the Galaxy and the Magellanic Clouds". Astronomy and Astrophysics 366 (2): 508–531. Bibcode:2001A&A...366..508V. doi:10.1051/0004-6361:20000022.
- Bibcode: 2001ASPC..233..133N
- Robert Burnham, Jr., (1978): Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System, (Revised and Enlarged Edition) Volume Two, New York, Dover Publications, pp. 772-773.
- Smith, Nathan (2007). "SN 2006gy: Discovery of the most luminous supernova ever recorded, powered by the death of an extremely massive star like Eta Carinae". arXiv preprint. Retrieved 2007-04-28.
- Thompson, Greg. "P Cygni profiles". Retrieved 5 December 2011.
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