P Cygni

P Cygni:

Magnitude 4.82

Variable type LBV (Luminous Blue Variable)

Spectral type B1la+



The hot blue star 34 Cygni, or P Cygni, shows a strong emission spectrum. It has sometimes been called a permanent nova, although that is not an accurate description. 

It is believed to have a strong solar wind in the form of a rapidly expanding gas shell.

Several of the spectral lines show an asymmetric behaviour. P Cygni is used as the model star for this behaviour, and the line profile is commonly called a P Cygni profileThe emission line peak is from light emission from the gas shell, energized by uv-radiation from the star. The dip is from the line of site absorption from the front part of the gas cloud, seen in the continuum of the star.


By measuring the blueshift of the typical absorption lines adjacent to the emission lines, I get about 3.5Å of shift for the 4861 line.

This transforms to a radial expansion velocity of about 0.0007 of the speed of light, or about 210 km/s (say +/- 50 km/s). This is without modelling the peaks. Just using the distance from max to min. In the example below on the 4861 peak, we have 4861.3Å-4857.8Å=3.5Å. I get similar results (210 km/s) on all 7 asymmetric lines.

P Cygni is believed  to be at some 5000 ly distance. It is a peculiar B star, designated spectral class B1-2 Ia-0ep, or B1 Iapeq. It is extremely luminous. There are ongoing discussions about a possible second companion to the star.

SPECTRUM: 6x60 sec exposure with spectrograph and 600 L/mm grating. 12" RC tetescope. The 3 images are from the same spectrogram data.