Before the moon became too bright in the sky at the beginning of July, I had time to observe one more variable star - AC Herculis. This is an RVa type star, the prototype being RV Tauri. Again, according to my Norton's Star Atlas RV Tauri stars are 'radially pulsating supergiants with alternating primary and secondary minima'. These types of variable star are subclassified into RVa and RVb stars; the former having constant mean magnitude (the class to which AC Herculis belongs) and the latter having a mean magnitude that varies by up to 2 mag. in periods of 600-1500 days. According to the BAAVSS AC Her varies from 6.8 to 9.0 and back again over a period of 75 days.
AC Her can be found near Flamsteed 106 and 109 in the constellation of Hercules. If you were to draw an imaginary line from the north celestial pole down through the star Vega in Lyra you would roughly come to the area of sky in which AC Her can be found. On the night of the 7th July 2022 I found the variable reasonably easily by using another atlas that I have (Storm Dunlop's Atlas of the Night Sky) and star hopping from the 'body' of Hercules with which I am familiar. The BAAVSS chart for AC Her is 048.04. On this evening, using my 7x50 binoculars, I could see star F on this map which is magnitude 8.8 and so my limiting magnitude was fainter than this. I determined that AC was fainter than star B (= 6.5 mag.) and C (= 6.9 mag.) but brighter than star E (= 8.2 mag.).
In fact at 23.36 UT I thought that AC was about equal in magnitude to star D which made it magnitude 7.4. Alternatively, I thought it was roughly halfway in brightness between stars C (= 6.9) and E (= 8.2) that is C(1)V(1)E. This made it magnitude (6.9 + 8.2)/2 = 7.55. Using both these estimates we get (7.4 + 7.55)/2 = 7.5 (to one decimal place). This was the estimate I recorded on the BAAVSS database.
All text and images © Duncan Hale-Sutton 2022
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