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

Mercury and the Pleiades

The BAA has Oberservers' Challenges and at the moment there is one that has been set by John Chuter to photograph the planet Mercury at its greatest elongation in April. Mercury reaches this point on April 29th when it will be within 1.4 degrees of the Pleiades (M45). We had some very nice clear skies two days ago on the 25th April and I thought I would go and see if I could spot the planet. The sun set about 8.15pm BST and I went out to look at the west-northwest horizon about half an hour after this. Initially, I couldn't spot the planet but after going in and coming out again I did see it about 10 degrees above the horizon (a bit more than a fist at arm's length). I began taking a whole series of pictures with my Nikon D90 but the best one was the very last I took at 9.29pm which incorporated the Pleiades nicely:-

I used a Nikkor VR 18-105 f/3.5-5.6 ED lens at 105mm. This was a single shot of 2.5s at f/5.6 ISO 1600. The Pleiades is the star cluster in the upper part of the picture and Mercury is above the tree line in the lower part. 

All text and images © Duncan Hale-Sutton 2022

The Sun on the 21st April 2022

On Thursday I took another picture of the sun using the same set up as I have described in an earlier post. I did make a mistake this time. I had pointed the telescope, as before, using the shadow cast by the sun before taking off the lens cap and improving the focus and alignment. However, I had left the star diagonal, eyepiece and, importantly, the eyepiece cap in place and when I reached down to remove it I had the brief shock of burning on my fingers. The sun, of course, had burnt a neat hole in the eyepiece cap and my fingers had touched molten plastic! It goes to show how careful you need to be. In fact, when I took the eyepiece out to check it over, I noticed that some of the inner plastic support for the lens had been melted, presumably when the sun was off centre. I am beginning to think I may need to stop down the objective lens to reduce the power of the sun's rays.

Anyway, here is my image (taken at 11:07 UT):-

The image has been rotated by 25 degrees so that North is at the top and East is at the left. I have also annotated the image to mark on the designated Active Regions (AR numbers). AR2993 and AR2994 are pretty big areas of sunspots. For example, the single connected region AR2993 is about 32 pixels across in this image whereas the full diameter of the sun is 1065 pixels. The sun is 864,000 miles in diameter and so this area of activity is roughly (32/1065) x 864,000 = 26000 miles across or 3.3 earth diameters! There is a lot of detail here. At full resolution (click on image) you can see that individual areas are made up of lots of much smaller spots.

For comparison, this is Solar Dynamics Observatory image taken on this day (Courtesy of NASA/SDO and the AIA, EVE, and HMI science teams):-

 

You can see how well the two images correlate.

All text and images © Duncan Hale-Sutton 2022 (except for the image from the SDO).