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Single star; early B-type supergiant (B1Ia); located in Cygnus; R.A. 20:17:47.200,
dec. +38:01:58.60; culmination late
P Cygni was first recorded in 1600 AD during an outburst lasting six years, when it reached third magnitude. After another outburst in 1654 or 5, also lasting several years, the star settled back to fifth magnitude a century later. Since then, "it has slowly increased its brightness (with many superimposed variations) by about 15 percent per century. The increase was not intrinsic, but was the result of cooling by 6 percent per century, which transfers progressively more of the star's ultraviolet light into the visible" (Kaler 2002, p. 139). It is now showing irregular photometric variations with an amplitude of ~0.2 mv.
|P Cygni is a type B supergiant, spectral type B1Ia, and among the most luminous stars in the Galaxy. However, the spectrum of P Cygni is most notable for the remarkable profile to which it has given its name, characterised by emission lines flanked by absorptions on their violet edges (see below).|
|In the past, P Cygni was classified in various imaginative ways; today it is regarded as the first discovered luminous blue variable (LBV).|
|Although detached shells of ejected gas exist around some
other well-known LBVs, such as h Carinae and AG Carinae, no
such feature has been reported in the optical spectrum of P Cygni. Instead, only a small
(< 8") circumstellar shell has been imaged at Ha
and [NII] wavelengths, apparently due to the continuous ejection of small shells with
constant average mass loss rate (see below). Extended (~30"
radius) thermal radio emissions have also been recorded.
(After Stahl 1989, p. 151.)
P Cygni spectra are characterised by emission lines flanked on their violet edges by absorptions. They are produced by an expanding envelope of gas, or stellar wind, as follows:
Other than that which is "blowing" straight towards or away from us, the gas streaming away from the star is travelling more or less perpendicular to our line of sight, producing emission lines which are not Doppler shifted. The gas which is travelling straight towards us, however, lies directly in front of the star, so producing an absorption feature in the stellar continuum, which is Doppler shifted to shorter wavelengths and appears on the violet edge of the emission lines. The gas streaming directly away from us on the far side of the star is not seen at all.
(After Kahler 1992, pp.193-194. See also Crowther 1997, pp. 51-57.)
At approximately two monthly intervals, P Cygni ejects shells of gas which increase the opacity of the star's envelope so that the bright photosphere is obscured and the star's brightness decreases. These shells are 'clumpy' so that, during their presence at minimum brightness, the photometric variations are larger than when the shells are absent, at maximum brightness.
(After de Groot 1989, p. 257.)
A borderline southern object; in areas with a very low northern horizon, P Cygni may possibly be visible very low down in the northern sky late August/early September.
Crowther 1997: Physical Properties of LBVs and Related Objects. In Nota, A.; Lamers, H.J.G.L.M. (eds) Luminous Blue Variables: Massive Stars in Transition. ASP Conference Series, vol. 120, pp. 51-57.
de Groot, Mart 1989: P Cygni: The Star that Started It All. In K. Davidson et al. (eds.) Physics of Luminous Blue Variables, pp. 257-258.
Gäng, Thomas; Leitherer, Claus; Wolf, Bernhard; Stahl, Otmar; Chapman, Jessica; van Gent, Jeroen; Lamers, H.J.G.L.M.; Scuderi, Salvatore 1997: Atmospheric Conditions in LBVs: First Results from a High-Resolution Optical Survey. In Nota, A.; Lamers, H.J.G.L.M. (eds) Luminous Blue Variables: Massive Stars in Transition. ASP Conference Series, vol. 120, pp. 110-112.
Kahn, F.D. 1989: The Formation of Shells in the Wind From P Cygni. In K. Davidson et al. (eds.) Physics of Luminous Blue Variables, pp. 177-183.
Kaler, James B. 1992: Stars. Scientific American Library.
Kaler, James B. 2001: Extreme Stars. Cambridge.
Kaler, James B. 2002: The Hundred Greatest Stars. Copernicus Books, 213 pp.
Lamers, Henny J.G.L.M. 1989: Mass Loss from Luminous Blue Variables. In K. Davidson et al. (eds.) Physics of Luminous Blue Variables, pp. 135-147.
Lamers, Henny J.G.L.M.; Cassinelli, Joseph P. 1999: Introduction to Stellar Winds. Cambridge.
Leitherer, Claus 1997: Mass Loss from LBVs: Observational Constraints. In Nota, A.; Lamers, H.J.G.L.M. (eds) Luminous Blue Variables: Massive Stars in Transition. ASP Conference Series, vol. 120, pp. 58-65.
Najarro, F.; Kudritzki, R.-P. 1997: The ISO-SWS Spectrum of P Cygni. In Nota, A.; Lamers, H.J.G.L.M. (eds) Luminous Blue Variables: Massive Stars in Transition. ASP Conference Series, vol. 120, pp. 105-109.
Stahl, O. 1989: Circumstellar Ejecta Around LBVs. In K. Davidson et al. (eds.) Physics of Luminous Blue Variables, pp. 149-155.
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