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)||4.82 (3 to 6)|
|U−B color index||-0.58|
|B−V color index||+0.42|
|Radial velocity (Rv)||-8.9 km/s|
|Proper motion (μ)||RA: -3.53 mas/yr
Dec.: -6.88 mas/yr
|Parallax (π)||0.32 ± 0.16 mas|
|Absolute magnitude (MV)||−7.9|
|Surface gravity (log g)||1.2 cgs|
|Rotational velocity (v sin i)||75 km/s|
Nova Cyg 1600, 34 Cyg, JP11 3218, TD1 26474, GSC 03151-03442, TYC 3151-3442-1, AG+37° 1953, 2MASS J20174719+3801585, ALS 11097, HD 193237, MCW 849, BD+37° 3871, Hen 3-1871, PLX 4837, CEL 5017, PPM 84645, P Cyg, RAFGL 5493S, GC 28218, HIP 100044, ROT 2959, GCRV 12673, HR 7763, SAO 69773, AAVSO 2014+37A.
P Cygni (34 Cyg) is a variable star in the constellation Cygnus. The designation "P" was originally assigned by Johann Bayer in Uranometria as a nova. 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 (1500-1800 parsecs) from the Earth. Despite this vast distance, it is visible to the naked eye in suitable dark sky locations. 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 P Cygni has been a fifth magnitude star, with only minor fluctuations in brightness. Today it has a magnitude of 4.8, irregularly variable by a few hundredths of a magnitude on a scale of days. The visual brightness is increasing by about 0.15 magnitude per century, attributed to a slow decrease in temperature at constant luminosity.
P Cygni has been called a "permanent nova" because of spectral similarities and the obvious outflow of material, and was once treated with novae as an eruptive variable; 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 Profile
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.
It has been proposed P Cygni's eruptions could be caused by mass transfer to a hypothetical companion star of spectral type B that would have a mass between 3 and 6 times the mass of our Sun and would orbit P Cygni each 7 years in a high eccentricity orbit. Infall of matter into the secondary star would produce the release of gravitational energy, part of which would cause an increase of the luminosity of the system.
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