The LBV "P Cygni" and "Feibelmann 1"
Feibelmann 1 is located directly next to a 4.8mag bright star named "P Cygni". As the name suggests, it belongs to the group of variable stars,
more precisely to the group of LBV - the "Luminous Blue Variables". These blue stars are very rare in our Milky Way. Only about 20 of them
are known. LBV are very massive and hot O- stars, which can change their brightness dramatically over a period of time. With masses from 30
to 150 solar masses they belong to the hypergiants and sometimes shine millions of times brighter than our Sun and reach bolometric magnitudes
of up to -11mag. The surface temperatures of these stars are between 30,000 and 50,000 Kelvin, while in the center unimaginable 800 million
Kelvin can prevail.
The distance to P Cygni is not yet known exactly and so the distances vary between 6,000 and 7,000 light-years. Therefore the luminosity, which
can be calculated from distance and apparent brightness, varies as well. At the moment we can say that the luminosity is somewhere in the range
of 600.000 to 900.000 suns. No matter which value is really correct - these are numbers you can hardly imagine. The surface of P Cygni is nearly
20,000 Kelvin hot, therefore it belongs to the spectral class B1, very close to the limit of the even hotter O- stars. With a mass of 30 solar masses
and a diameter of 105 million kilometers the LBV is also a truly gigantic star, whose surface would reach the orbit of Mercury.
About the star cluster "Feibelmann 1" I couldn't really find much information. Its distance is given with 7,500 light-years, which would imply that
P Cygni indeed belongs to this cluster. The age of about 8 million years would also fit well to this assumption, because after this time span a
"Luminous Blue Variable" would still exist.
It also seems that "Feibelmann 1" and the nearby cluster "IC 4996" form a kind of double cluster. The two clusters are 28' apart on the sky.
Converted to the distance this results in a distance of 61 light years. So this could be plausible.
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At a low magnification of 120x "P Cygni" catches the eye immediately. But strangely it does not glow bluish-white at all, but in a warm pastel
yellow. It is assumed that the light is slightly reddened by dust masses in front of it. After letting the photons of this monstrous LBV hit my retina
for a while, I went in search of "Feibelmann 1" - this ominous star cluster trying to hide in the bright glow of "P Cygni".
Armed with a DSS printout, a high precision printout from Guide and a magnification of 380x I got close to the cluster. It was not so easy to
catch the many faint stars next to the glaringly bright star with indirect vision and to curl them on the map after a successful sighting. 5mag bright
stars and 20 inch aperture do not get along so well, so that one permanently loses the dark adaptation. Although the seeing was quite good, the
visibility was rather poor, so I only got down to a limiting magnitude of about 16.3mag. In better conditions I surely would have caught a few
more stars than the 40 I saw (of which I'm sure some are normal field stars). But I think that I was able to catch the star cluster "Feibelmann 1"
successfully.
