Peripatus Home Page  pix1Black.gif (807 bytes)  Astronomy Page >> Some Southern Objects >> P Cygni Updated: 15 Jun 2003 

P Cygni


Shim1Pel.gif (799 bytes) P Cygni Shim1Pel.gif (799 bytes) Bayer designation (1603) Shim1Pel.gif (799 bytes)
Shim1Pel.gif (799 bytes) 34 Cygni Shim1Pel.gif (799 bytes) Flamsteed designation (1712, 1725) Shim1Pel.gif (799 bytes)
Shim1Pel.gif (799 bytes) HR 7763 Shim1Pel.gif (799 bytes) Harvard Revised Photometry designation (1879+) Shim1Pel.gif (799 bytes)
Shim1Pel.gif (799 bytes) HD 193237 Shim1Pel.gif (799 bytes) Henry Draper Catalogue (1918+) Shim1Pel.gif (799 bytes)
Shim1Pel.gif (799 bytes) SAO 69773 Shim1Pel.gif (799 bytes) Smithsonian Astrophysical Observatory (1966+) Shim1Pel.gif (799 bytes)
Shim1Pel.gif (799 bytes) 100044 Shim1Pel.gif (799 bytes) Hipparcos catalogue (1997) Shim1Pel.gif (799 bytes)

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 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.)

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
mv 4.82  
Mv -8.6 Kaler 2001, p. 101
Mbol -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  
Hb 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
Teff 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.


 Peripatus Home Page  pix1Black.gif (807 bytes)  Astronomy Page >> Some Southern Objects >> P Cygni Contact me.