The star is located about 5,000 to 6,000light-years (1,500–1,800parsecs) from 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 3rdmagnitude. It was first observed on 18 August (Gregorian) 1600 byWillem Janszoon Blaeu, a Dutch astronomer, mathematician and globe-maker. Bayer's atlas of 1603 assigned it themiscellaneous label P and the name has stuck ever since.[12] 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.[13] The visual brightness is increasing by about 0.15 magnitude per century, attributed to a slow decrease in temperature at constant luminosity.[14]
P Cygni has been called a "permanent nova" because of spectral similarities and the obvious outflow of material, and was once treated withnovae as aneruptive variable; however, its behaviour is no longer thought to involve the same processes associated with true novae.[15]
Avisual bandlight curve for P Cygni. The main plot is fromAAVSO data.[16] The inset plot, adapted from de Groot (1988),[17] shows the variability for the first 400 years after the star's discovery.
P Cygni is widely considered to be the earliest known example of aluminous blue variable. However, it is far from a typical example. Typically, LBVs change in brightness with a period of years to decades, occasionally hosting outbursts where the brightness of the star increases dramatically. P Cygni has been largely unvarying both in brightness andspectrum since a series of large outbursts in the 17th century. Similar events have been seen inEta Carinae and possibly a handful of extra-galactic objects.[18]
P Cygni does show evidence for previous large eruptions around 900, 2,100, and possibly 20,000 years ago. In more recent centuries, it has been very slowly increasing in visual magnitude and decreasing in temperature, which has been interpreted as the expected evolutionary trend of a massive star towards ared supergiant stage.[18]
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 asupernova. The recent supernovaSN 2006gy was likely the end of an LBV star similar to P Cygni but located in a distant galaxy.[19] P Cygni is thought to be in the hydrogen shell burning phase immediately after leaving the main sequence.[18]
It has been identified as a possibletype 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).[20]
P Cygni's characteristic and eponymous line profile forH-α
P Cygni gives its name to a type ofspectroscopic feature called a P Cygni profile, where the presence of both absorption and emission in the profile of the samespectral 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 theblueshifted absorption lobe is created where the radiation passes through circumstellar material rapidly expanding in the direction of the observer. These profiles are useful in the study ofstellar winds in many types of stars. They are often cited as an indicator of aluminous blue variable star, although they also occur in other types of star.[18][21]
In P Cygni itself, the size of the stellar windH-alpha emission region is5.64±0.21milli-arcseconds.[13] At the distance of 1,600parsecs this is a physical size of approximately 25 stellar radii.
It has been proposed P Cygni's eruptions could be caused bymass transfer to a hypotheticalcompanion star ofspectral type B that would have amass between 3 and 6 times the mass of the Sun and would orbit P Cygni each 7 years in a higheccentricityorbit. Infall of matter into the secondary star would produce the release ofgravitational energy, part of which would cause an increase of the luminosity of the system.[22] A companion star to P Cygni was indeed revealed in 2021 usinginterferometry, which when discovered had an angular separation of13.0±0.1 mas from the primary. It is 4.3 magnitudes fainter than P Cygni.[23]
^Ducati, J. R. (2002). "VizieR On-line Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system".CDS/ADC Collection of Electronic Catalogues.2237: 0.Bibcode:2002yCat.2237....0D.
^abcSamus, N. N.; Durlevich, O. V.; et al. (2004). "VizieR Online Data Catalog: Combined General Catalogue of Variable Stars (Samus+ 2004)".VizieR On-line Data Catalog: II/250. Originally Published in: 2004yCat.2250....0S.2250: 0.Bibcode:2004yCat.2250....0S.
^Skiff, B. A. (2014). "VizieR Online Data Catalog: Catalogue of Stellar Spectral Classifications (Skiff, 2009-2016)".VizieR On-line Data Catalog: B/Mk. Originally Published in: Lowell Observatory (October 2014).1.Bibcode:2014yCat....1.2023S.
^abSmith, L. J.; Crowther, P. A.; Prinja, R. K. (1994). "A study of the luminous blue variable candidate He 3-519 and its surrounding nebula".Astronomy and Astrophysics.281: 833.Bibcode:1994A&A...281..833S.
^abcNajarro, F. (2001). "Spectroscopy of P Cygni".P Cygni 2000: 400 Years of Progress.233: 133.Bibcode:2001ASPC..233..133N.
^Najarro, F.; Hillier, D. J.; Stahl, O. (1997). "A spectroscopic investigation of P Cygni. I. H and HeI lines".Astronomy and Astrophysics.326: 1117.Bibcode:1997A&A...326.1117N.
^Lamers, H. J. G. L. M.; De Groot, M. J. H. (1992). "Observed evolutionary changes in the visual magnitude of the luminous blue variable P Cygni".Astronomy and Astrophysics.257: 153.Bibcode:1992A&A...257..153L.
^Smith, Nathan; Li, Weidong; Foley, Ryan J.; Wheeler, J. Craig; Pooley, David; Chornock, Ryan; Filippenko, Alexei V.; Silverman, Jeffrey M.; Quimby, Robert; Bloom, Joshua S.; Hansen, Charles (2007). "SN 2006gy: Discovery of the Most Luminous Supernova Ever Recorded, Powered by the Death of an Extremely Massive Star like η Carinae".The Astrophysical Journal.666 (2):1116–1128.arXiv:astro-ph/0612617.Bibcode:2007ApJ...666.1116S.doi:10.1086/519949.S2CID14785067.
