A Trio of Special Stars

October 10, 2017

P_Cygni_annotate_EPOD (1)

Photographer: Greg Parker
Summary Author: Greg Parker

I’m particularly interested in ultra deep-red carbon stars and have managed to image quite a number of them over the years. Additionally, I’m also interested in very large, and or very luminous stars, which includes a hypergiant, luminous blue variable (LBV) known as P Cygni, one of the most luminous stars in the galaxy.

As shown above, P Cygni (center) as well as carbon stars BC Cygni (lower left) and SAO 69636 (over the Crescent Nebula) all appear here within the same frame – in the direction of the constellation of Cygnus. P Cygni is so massive and energetic that it’ll exhaust its nuclear fuel relatively quickly. In its short lifetime of just a few million years it will end its life in a supernova explosion.

I was previously aware of BC Cygni but didn't think at the time I made this image that it was particularly important. But of course it was. Coming back to the image several months after taking it and tracking BC Cygni down using a planetarium program, I discovered that it’s a red supergiant star, with a (variable) magnitude near 8.42. The magnitude of BC Cygni varies from +9.0 to + 10.8 over a period of 720+/-40 days. The B-V color index of B C Cygni is a massive 3.64, making it very red indeed, as can be seen in the image. Although it doesn't appear particularly large here, it’s in fact between 1140 and 1230 times larger than our Sun. So if it was placed in the position of our Sun it would actually encompass the orbit of Jupiter.

SAO 69636, also known as R S Cygni, is also a variable star with a magnitude of 7.61 and a B-V index of 3.59. Thus it’s red, though not quite as red as B C Cygni. I find it quite remarkable that three such exceptional stars are present in such a relatively small region of space.

Photo Details: Image was taken using the mini-WASP array at the New Forest Observatory in the U.K., using 18 sub-exposures at 10-minutes per sub, with the image sensor at -20C.