|Right Ascension|| 20 : 17 : 47.1 (h:m:s)
|Declination|| +38 : 01 : 59 (deg:m:s)
|Distance|| 7,000 (ly)
|Visual brightness|| 3.0 .. 6.0 (mag)
|Spectral type|| B2pe
P Cygni, also known as 34 Cygni, "Nova Cygni 1600" or "Nova Cygni #1", is one of the most remarkable variable stars in the sky, and the third which became known to modern astronomers. It was first noted in 1600 in a place where no star had been recorded before, as a 3rd magnitude star. According to Burnham's Celestial Handbook and Robert Garfinkle's book "Star Hopping", the first well-documented observations were made by the Dutch astronomer and mathematician Willem Janszoom Blaeuw (1571-1638) on August 8, 1600. On a globe made by Blaeuw around 1840 which is now stored in a Prague museum, he did the following inscription (from Burnham):
``The new star in Cygnus that I first observed on August 8, 1600, was
initially of 3rd magnitude. I determined its position .. by measuring its
distance from Vega and Albireo. It remains in this position but now is no
brighter than 5th magnitude.''
Within the next few years, the star then faded below naked-eye visibility, but returned to magnitude 3.5 in 1655, where it remained until 1659. It faded again below 6th mag, and rose once more in 1665. After some fluctuations, it became steady at about magnitude 5 around 1715, and during the last 200 years, has oscillated in brightness around this value.
The brightness bursts are now explained by the assumption that the star is throwing off shells of gas. When looking at the spectrum of this star, a curious profile of the absorbtion lines can be seen: Instead of of a more or less symmetric line profile a broad trough with an emission peak at the red side of the trough.
Investigations with satellites showed that below 2000 Angstroem many O and B supergaints have this kind of lines in their spectra. It is something so typical that P Cygni gave name to a subclass of stars, the P Cygni stars.
Nevertheless, the novalike flares classify P Cygni as another type of stars, sometimes called "permanent nova", and similar to Eta Carinae in our Milky Way Galaxy, S Doradus in the Large Magellanic Cloud (after which this class of variables has been named) and the so-called Hubble-Sandage variables in some more remote galaxies.
The distance of P Cygni is not well known, but has been estimated as 7000 light years by Burnham, putting its absolute brightness to -8.9 magnitudes, one of the brightest stars in our galaxy.