DSO of the Month
: Cygnus X-1
AKA: V1357 Cyg; HD 226868
Position: 19 hrs 58 min 21.7 +35 degrees 12 min 07 sec
Due south at 01.23 (BST) on 16 July 2021
Cygnus X-1 (the lower of the two stars)
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
A couple of months ago, I showed you how to find a quasar from your back garden. This month I am going to show you how to find a black hole. Well obviously I cannot show you a black hole by definition, but if a stellar mass black hole is in a binary system with a star, it will show up by accreting mass from its unfortunate neighbour and you will be able to see the other star anyway. To be pedantic, this is technically a star of the month rather than a DSO of the month, but I feel it deserves to be considered a DSO as it is no longer a star. The accretion disk is extremely energetic and has jets streaming out of it, like a quasar or a galaxy such as M87 with an active supermassive black hole. In fact it is so hot, that it is luminous in the X-ray region rather than visible light (hence the name X-1). Fortunately for us, X-rays are blocked by the Earth’s atmosphere, but astronomers started in the early 1960s to send rockets carrying Geiger counters up into the upper atmosphere to look briefly for X-ray sources. One of the most obvious of these early X-ray sources was in Cygnus and when it was discovered in 1964, it was called XR-1 (later changed to X-1), but it was not associated with any obvious visible or radio source. After the Uhuru X-ray satellite was launched in 1970, it was noticed that Cygnus X-1 had fluctuations which implied it had a diameter smaller than the Sun and it was then connected to a specific star, a blue supergiant. Soon it became clear that the hidden companion to the visible star was massive (it is now known to be 21 solar masses) and astronomers began to wonder if it could be a black hole, a completely theoretical concept at the time. Famously Stephen Hawking made a bet with Kip Thorne in December 1974 that it was not a black hole; Hawking finally conceded the bet in 1990.
To “see” this famous X-ray source and black hole, which is 6,100 light years distant, look for a 8.9 magnitude star (to be precise HD 226868) just 26 arc minutes (or roughly the diameter of the full moon) below Eta Cygni, which is a 3.9 magnitude star between Sadr (Gamma Cygni) and Albireo (Beta Cygni). Between Eta Cygni and Cygni X-1 is another star, HD 189148 (mag 7.3). As you can see in the above image there is a 10th magnitude star just above Cygnus X-1.
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