P Cygni

	P Cygni		Bayer designation (1603)
	34 Cygni		Flamsteed designation (1712, 1725)
	HR 7763		Harvard Revised Photometry designation (1879+)
	HD 193237		Henry Draper Catalogue (1918+)
	SAO 69773		Smithsonian Astrophysical Observatory (1966+)
	100044		Hipparcos catalogue (1997)

Synopsis

Single star; early B-type supergiant (B1Ia); located in Cygnus; R.A. 20:17:47.200, dec. +38:01:58.60; culmination late August/early September; magnitude 4.8; distance 2150 pc (approx 7000 light years).

Description

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 m_v.

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.)

Property	Value	Notes/References
R.A.	20h 17m 47.200s
Declination	+38º 01' 58.60"
Precession in RA	2.21 s/a for 10 years
Precession in dec.	11.3 arcsec/a for 10 years
Proper Motion RA	-3 mas/yr	Includes cos(dec) term.
Proper Motion dec	-6 mas/yr
Radial Velocity	-9 km/s
Parallax	0 mas
Distance	1820 pc (log d = 3.26) 2150 pc (7000 ly) 1840-2150 pc (6000-7000 ly)	Lamers 1989, p. 138, table 1 Kaler 2001, p. 101 Kaler 2002, p. 139
m_v	4.82
M_v	-8.6	Kaler 2001, p. 101
M_bol	-9.9	Lamers 1989, p. 138, table 1
M/M_¤	35 30 50-60	Kahn 1989, p. 178 Lamers & Cassinelli 1999, p. 197, table 8.1 Kaler 2002, p. 139
Mass Loss Rate (M_¤/yr)	2 x 10^-5 3 x 10^-5 1.5 x 10^-5 1.5 x 10^-5	Leitherer 1997, p. 60, table 2 Najarro & Kudritzki 1997, p. 105 Gäng et al. 1997, p. 111 Lamers & Cassinelli 1999, p. 197, table 8.1
v_¥	185 km/s 185 km/s 210 km/s	Crowther 1997, p. 53 Najarro & Kudritzki 1997, p. 105 Lamers & Cassinelli 1999, p. 197, table 8.1
L/L_¤	750,000 630,000 (log L = 5.8) 560,000 720,000 500,000-900,000	Kahn 1989, p. 178 Leitherer 1997, p. 60, table 2 Najarro & Kudritzki 1997, p. 107, table 1 Lamers & Cassinelli 1999, p. 197, table 8.1 Kaler 2002, p. 139
B-V	+0.42
U-B	-0.58
V-R	+0.54
R-I	+0.26
b-y	+0.353
m1	-0.046
c1	-0.059
H_b	2.612
Spectral Type	B1Ia+	Lamers & Cassinelli 1999, p. 197, table 8.1
R/R_¤	76 76 76	Lamers 1989, p. 138, table 1 Najarro & Kudritzki 1997, p. 105 Lamers & Cassinelli 1999, p. 197, table 8.1
T_eff	19,300 20,000 18,100 19,300 18,000-19,000	Lamers 1989, p. 138, table 1 Kahn 1989, p. 178 Najarro & Kudritzki 1997, p. 105 Lamers & Cassinelli 1999, p. 197, table 8.1 Kaler 2002, p. 139
Rotational Velocity	75 km/s
Period	50 days	de Groot 1989, p. 258
Table 1: Known Parameters for P Cygni. (After Ochsenbein & Halbwachs 1987 except where otherwise noted.)

Discussion

P Cygni Spectra

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.)

Mass Loss

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.)

Observation

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.

References

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.

Ochsenbein F.; Halbwachs J.L. 1987: Le Catalogue des Etoiles les Plus Brillantes (Catalogue of the Brightest Stars). Bull. Inform. CDS 32, 83. NASA Astronomical Data Centre, catalogue 5053A.

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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