TX and AH Draconis

Three days ago (the 19th July 2022) we had some brief clear weather close to midnight and I was able to observe TX and AH Dra. Initially, I did look for RY and Z UMa in Ursa Major (The Big Bear) but it is now low down in the north and there was too much atmospheric extinction and twilight at that altitude for me to be able to make a decent estimate. The moon wasn't a problem at this time as it was two days from last quarter and hadn't risen.

Beginning with TX Dra which is easier to find (being close to Eta Dra) I found that I could, with my 7x50 binoculars, see stars K (=7.0 mag.), N (=7.7 mag.) and P (=8.4 mag.) on BAA chart 106.03. This meant my limiting visual magnitude was fainter than 8.4. At 22:34 UT I estimated that TX was fainter than K but brighter than N but closer in brightness to N. So my estimate was K(2)V(1)N which made it magnitude 7.5 (to one decimal place).

Going on to AH Dra, I found that the star was fainter than the star labelled 1 (=7.1 mag.) but brighter than the star labelled 8 (=8.4 mag.) but not much brighter than 8. So my estimate was 1(3)V(1)8 which made it magnitude 8.1 (to one decimal place).

Both these estimates were in good agreement with other observers from the BAA and AAVSO.

All text and images © Duncan Hale-Sutton 2022

AC Herculis - an RVa variable star

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

CH Cygni - another variable to monitor

Whilst we still have some dark skies before the moon interferes again, I thought I would try and monitor a different variable star. With some advice from the BAA I have decided to observe CH Cygni. The Variable Star Section says that this is a ZAND+SR type variable star with a magnitude range of between 5.6 and 10.5. According to my Norton's Star Atlas a ZAND star is a type of cataclysmic variable consisting of a close pair of binary stars where one hot star is exciting the envelope of a cooler one. Note that the abbreviation of ZAND comes from Z And, the first star of this type to be noted. These are also called symbiotic stars. I guess that the +SR means that the cool star in the binary is also a Semi Regular pulsating star. Unlike pulsating stars (of which Z and RY UMa and TX and AH Dra are examples) cataclysmic variables do not have a regular variation in their brightness, so their magnitude can alter from one night to the next in a chaotic fashion and can thus be observed on a regular basis.

The finder chart for CH Cyg 089.04 (pdf) can be found on the BAA's Variable Star Section website. The three stars that aid you to find CH are Theta, Iota and Kappa Cygni, which form the more northerly wingtip of the Swan. On the 3rd July 2022 I went out about midnight (BST) to search for CH using my 7x50 binoculars. It was relatively easy to find as it is not far from the star marked A on the chart. I could make out star F with my bins which meant that my limiting magnitude was fainter than magnitude 8.5. At 23:26 UT I estimated that  CH was fainter than star A (magnitude 6.5) but brighter than star D (magnitude 8.0). I thought that the star was much closer in brightness to A than it was to D and to put it in numerical terms it was about one step from A and 3 steps from D, that is A(1)V(3)D. The difference in magnitude between A and D is 8.0 - 6.5 = 1.5 magnitudes. Dividing this by 4, each step corresponds to 0.375 mag., making CH 6.5 + 0.375 = 6.875 or 6.9 (to one decimal place). I verified this by comparing it to stars A (=6.5 mag.) and W (=7.3 mag.) finding it to be roughly between these two stars in brightness. We have that (6.5 + 7.3) = 6.9. This estimate compares favourably with other observations on the BAA database.

All text and images © Duncan Hale-Sutton 2022

More observations of RY and Z UMa and TX and AH Dra (30th June/1st July 2022)

On Thursday night (the 30th June) we had a clear patch of weather which enabled me to make some more observations of the four variable stars I have been monitoring. It had rained heavily in the afternoon (in the form of a few torrential showers) but as evening came on the clouds began to clear away. We are still in that period of the summer when the sky never gets completely dark but going out about midnight, it is dark enough to see these 7th and 8th magnitude stars. Also the moon wasn't going to be a problem, it being only one day past new. I also had the benefit of some new 7x50 binoculars and this has made a bit of a difference in these conditions.

Starting with Z UMa again, on the chart I could make out star H quite clearly so the limiting visual magnitude was fainter than 8.7. I had no problem finding the star and it had faded since I last saw it. I determined that at 23.25 UT it was brighter than H (mag. 8.7) but fainter than D (mag. 7.9) but closer in brightness to D than H. The way of describing this technically was to record this as D(1)V(2)H. This means that the star was one "point" away from D but two "points" away from H. The difference in brightness between D and H is 0.8 magnitudes, so each point is approximately 0.8/3 = 0.27 magnitudes making Z 7.9 + 0.27 = 8.17 mag., or 8.2 to one decimal place.

RY UMa had also faded a bit since my last observation at the beginning of the month. At 23.40 UT it's brightness was between the star labelled 2 (7.4) and the star labelled 4 (7.7) but closer to 4. I recorded it as 2(2)V(1)4 making it magnitude 7.6.

Notice these times are well past midnight in British Summer Time, so I was feeling a bit tired. Nevertheless, I thought I ought to have another go at TX and AH Draconis. I started with TX as this is close to the star Eta Dra. At 00.08 UT (on the 1st July 2022) TX was brighter than N (7.7) but fainter than K (7.0). It was sort of midway in brightness between the two but marginally closer to K. I decided to record it as K(3)V(4)N which made it magnitude 7.3. 

Finally, at 00.29 UT AH was fainter than the star labelled 1 (7.1) but brighter than the star labelled 8 (8.4). In fact, it was close in brightness to the star labelled 6 (7.8). I recorded it as 1(1)V(1)8 which made it magnitude 7.8 (to one decimal place).

Comparing my observations to other observers at the BAA, the only estimate that caused me some concern over accuracy was that for TX Dra. The last few observations of this star had its magnitude around 8.0. However, looking at the AAVSO my estimate of 7.3 is close to what other observers at this association are seeing at the moment. All my observations have been loaded into the BAA database.

All text and images © Duncan Hale-Sutton 2022