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

So what sort of variable star is Z UMa?

I have recently been making observations of the variable star Z Ursae Majoris. I have described the location of this star in a previous post. On their beginners webpage, the BAA describe Z UMa as a type SRb star with a visual range of variability of between 6.2 and 9.4 magnitudes over a period of 196 days. According to my Norton's Star Atlas SR stands for Semi Regular and it is a class of red giant star which has noticeable periodic fluctuations in its brightness but with some irregularities. The sub-class "b" indicates the periodicity is poorly defined (as opposed to sub-class "a" where the periodicity is persistent). This type of star is known as an intrinsic variable - its variation is caused by physical processes that are occurring within the star as opposed to extrinsic variables where the variation is caused by a geometric effect (such as eclipses between two stars).

Semiregular stars are pulsating variables. The changes in brightness are caused by size and shape changes of the stars themselves. The mechanism is relatively well understood (pdf) for some types of pulsating stars (such as Cepheid variables) but is less well understood for long period variables such as the semiregular stars. In fact, for SRb stars there is often more than one period of oscillation. In June 2020 John Greaves published a paper that looked at about 50 years of data from the BAA Variable Star Section database and concluded that from 1968 until about 1995 the main period of variability was 194.0 days but there was another contributory pulsation period of 204.8 days. This is broadly similar to studies made of data at the AAVSO.

Both SRa and SRb stars have late-type spectral classes of M, C or S and are red giants or supergiants. These stars are highly evolved stars that approaching the end of their lives. Z UMa has a M5IIIe spectrum (pdf) typical of a cool red giant with molecular bands (from diatomic Titanium Oxide) and emission lines (weak hydrogen delta and gamma lines). The presence of these metallic oxides condensing out in the outer layers of of stars like Z UMa complicates the observation of these stars because these molecules absorb visual band light and this exaggerates the amplitude of the variations to visual observers.

Interestingly, there is some indication that amplitude of variation may be increasing in recent years (see the paper by Greaves). This means that range of apparent magnitude from minimum to maximum is getting larger. Also from 1995 the data from the BAA seems to indicate that the star is settling on a single pulsation period of 189.0 days. Does this mean that Z UMa is becoming more like a SRa variable or even a Mira variable? It seems to me that there is a lot of reasons to continue following the variability of this star.

Variable stars Z and RY UMa - 1st June 2022

We had some nice clear weather last night so I was out in the garden again trying to observe the variable stars Z and RY Ursae Majoris (see my previous post for full details). The constellation of Ursa Major (The Great Bear) is now on the western side of the pole star and is heading downwards. The big problem about observing at this time of year is that the sky doesn't get properly dark due to the sun being less than 18 degrees below the northern horizon. At least the moon was out of the way due to it being two days past new.

I found the pattern of stars again, using my 8x24 binoculars, that would lead me to Z UMa. At 11:42 BST I could just about make out star E on BAA chart 217.02 which meant that the limiting magnitude was fainter than magnitude 8.4. With a bit of concentration I figured out that Z at 23:09 UT was midway between B and C in brightness putting it at magnitude 7.4. You can view what other observers from the BAA estimate the magnitude of Z to be by looking at their database. If you click on 'Review Observations' and then select object Z UMa you can produce a 'Data Table' (try limiting the start and end dates from 01-05-2022 to 30-06-2022). The most recent observations are at the top of the table. My estimate seems to be in the right ball park for the 01-06-2022.

I then went on to look at RY UMa. I estimated at 23:22 UT that RY was midway in brightness between stars marked as 1 and 4. Star 1 is magnitude 6.7 and 4 is 7.7 which is quite a big difference (1 magnitude). But halfway in brightness would make RY magnitude 7.2 which is a bit brighter than the star labelled 2 which is what I saw. Again this estimate agrees well with other values on the BAA VSS database.

All text and images © Duncan Hale-Sutton 2022

TX and AH Draconis

I thought I would have another go at some more variable stars on Monday (the 25th April) to see if I could come up with some more magnitude estimates. The BAA has some other springtime suggestions for beginners with binoculars. I decided to have a go at AH Draconis (AH Dra) which is on chart 106.03. Also on this chart is the variable TX Draconis (TX Dra) which I thought I would do as well. TX Dra is marked in my Atlas and isn't far from Eta Dra. Draco, the Dragon, is a sprawling constellation that sort of encircles the Little Bear, Ursa Minor. At this time of year Draco, which is circumpolar from my latitude, is climbing the sky east of the North Celestial Pole. From the Pole Star, if you draw a line through the first couple of stars of the Little Bear's tail and continue it on towards the east you will come to Eta Dra.

TX Dra is southeast of Eta and forms a sort of pentagonal with four other stars which include the star labelled K on the chart and the variable WW Dra. There are 7 comparison stars labelled R, S, K, T, N, P and U. These seemed a lot more spread out on the chart than the ones for Z and RY UMa. This does make it more difficult to make a comparison between stars as you have to scan across the sky from one star to another. However, I was fortunate that TX was apparently equal in brightness to the star labelled K which is nearby. So at 21:27 UT my estimate of TX's magnitude was 7.0. If you again go to the American Association of Variable Star Observers' website you can plot out the latest estimates of the visual magnitude for this star. Just put in TX Dra into their 'Pick a star' box and 'Plot a light curve'. The Julian Date for the 25th April 2022 at 21:27 is 2459695.39375. You can see that my estimate seems to be pretty good (click on any of the black open circle data points around this date and you will see estimates around this value).

The moon on this date was two days past last quarter and so was not a problem. The sky was quite dark and I could identify star P at 21:13 UT which is magnitude 8.4, so the limiting magnitude for my 8x24 binoculars was fainter than this. I then went on to look at AH Dra. There are some brightish stars nearby which make locating AH relatively easy. These brightish stars form a sort of isosceles triangle with Eta and Nu Dra with these latter stars forming the base. I found the comparison with other stars (labelled 7, 1, 2, 6, 8, 5, and 9) difficult because of their being scattered but at 21:48 UT I thought that AH was roughly equal in brightness to the star labelled 2 which put it at magnitude 7.3. Again by plotting the light curve for this star on the AAVSO website. It is difficult to tell at the moment as their is sparsity of data, but I may be a bit off what other observers are reporting as it was fainter than this.

All text and images © Duncan Hale-Sutton 2022