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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. Located about 5,000 to 6,000 light-years (1,500–1,800 parsecs) from Earth, 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.

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 profile of the same spectral line indicates the existence of a gaseous envelope expanding away from the star. The emission line arises from a dense stellar wind near to the star, while the blueshifted absorption lobe is created when the radiation passes through circumstellar material rapidly expanding in the direction of the observer. 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, although they also occur in other types of star.

The size of the stellar wind H-alpha emission region is $5.64$ milli-arcseconds. At the estimated distance of 1,700 parsecs this is a physical size of approximately 26 stellar radii.

Variability & Eruptions
P Cygni was unknown until the end of the 16th century, when it suddenly brightened to 3rd magnitude. It was first observed on August 18th (Gregorian), 1600, by Willem Janszoon Blaeu, a Dutch astronomer, mathematician and globe-maker. The first well documented account of its discovery is on a globe made by Blaeu, in Amsterdam, in the year 1602, which is currently being housed at Skokloster Castle, in Sweden. After six years, in 1626, the star faded below naked-eye visibility. It brightened again in 1655, but had faded by 1662. Another outburst took place in 1665; this was followed by numerous fluctuations. These brightenings were eruptions which are typical of LBVs, most famously Eta Carinae underwent similar events. Additionally there is evidence that P Cygni experienced large eruptions around 900, 2,100, and possibly 20,000 years ago. 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 the 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.

Since 1715 P Cygni has been in a period of quiescence at fifth magnitude, with only minor fluctuations in brightness. Today it has a magnitude of 4.8 and is irregularly variable by a few hundredths of a magnitude on a scale of days. The visual brightness is estimated to be increasing by about 0.15 magnitude per century, attributed to a slow decrease in temperature at constant luminosity. This brightening has been interpreted as the expected evolutionary trend of a massive star towards a red supergiant stage.

P Cygni has been called a "permanent nova" because of spectral similarities and 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
P Cygni is considered to be the earliest known example of a luminous blue variable, a very rare type of evolved massive star. Only ~20 LBVs are known in the Milky Way and the Large Magellanic Cloud. Typically, LBVs change in brightness with a period of years to decades, occasionally hosting outbursts where the brightness of the star increases dramatically. However, P Cygni has been largely unvarying both in brightness and spectrum since a series of large outbursts in the 17th century. Similar events have been seen in Eta Carinae and possibly a handful of extra-galactic objects.

Evolution
Luminous blue variables 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 the 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 the 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 is thought to be in the hydrogen shell burning phase immediately after leaving the main sequence.

It has been identified as a possible type IIb supernova candidate in modelling of the fate of stars 20 to 25 times the mass of the Sun (with LBV status as the predicted final stage beforehand).