Variable Stars 2018

Unless stated otherwise all variable star charts or phase diagrams were constructed from data taken with a William Optics Megrez 90 telescope with a 0.8x focal reducer and with an Atik 460EX CCD camera through a Johnson V photometric filter. Exposures are chosen to avoid saturating the variable star or any of the comparison stars.

The target star is located using SGPro software with plate solving. The data is analysed with the free software Muniwin and the final chart produced in Excel. To produce a phase diagram I have used Peranso.

Finder charts have usually been obtained from the AAVSO website together with details of suitable comparison stars. All results have been submitted to the BAA and AAVSO.

Eclipsing Binary star AB Cassiopeia

The chosen exposure time for these readings was 10s. The gap in the readings is where I performed a meridian flip after which the gear did not behave for a while.

This chart shows the primary eclipse.

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Eclipsing Binary Star AD Andromeda

The chosen exposure time for these readings was 20s. There was a near full Moon and the target star was of lowish altitude which may explain the scatter of the readings. Using the program Peranso with these and readings taken last year I determined the period of this binary to be 0.9862 days which agrees with the value given by aavso.

Strangely the time of the primary minimum was about an hour different from the Krakow prediction of 22:54 UT. On further investigation it turns out that the Krakow database predictions for this star is based on 15 year old data and an hours difference now could be produced with just one second in the time period. This data has been submitted to them for possible updating of their predictions.

Pulsating variable star YZ Boo. All of my previous variable star measurements have been of eclipsing binary stars where the change in brightness is due to one star coming in front of the other. YZ Boo is a single star that pulsates radially and so causing a brightness fluctuation.

The process behind this is essentially the same as in Cepheid variables. Main sequence stars are in equilibrium. There is an outward force due to the internal pressure which depends on the temperature and an inward force due to gravity. The driving mechanism behind "self-excited" oscillations is a special region of the stellar interior where atoms of either hydrogen or helium transition from partly to completely ionized. If the star is compressed, the ionization fraction of these regions increases, raising the opacity of the material and blocking the luminous energy trying to escape from the interior. The increased heat and pressure built up in this layer push the outer layers of the star outward. As these outer layers fall back inward again under the force of gravity, the ionization region gets compressed again, restarting the cycle. The variation in brightness is caused by changes in temperature and radius caused by these motions. (Thanks to the AAVSO website for this explanation.)



Pulsating variable star YZ Boo.

Eclipsing Binary star AI Draco

AI Draco is an eclipsing binary star of type EA. That is that it is similar to the star Algol in that the two stars are sufficiently far apart that apart from the gravity that keeps them in orbit around each other there are no other significant effects. The exposure time was 8 seconds (repeatedly)

V972 Cepheus

This is my first HADS star light curve.

The term HADS means High Amplitude Delta Scuti stars. This is a pulsating star of higher amplitude than the normal Delta Scuti stars. There have been a number of problems with this set of measurements. Firstly the gap in the readings was entirely due to my own stupidity. I hadn't noticed that the readings had stopped as it had reached the number that I had set. Secondly on the the aavso finder chart there were only two comparison stars indicated and they were 3 to 4 magnitudes brighter than the variable so I set an exposure to not saturate the CCD for those stars. This meant that the variable was coming out rather faint. The software chucked out all the readings possibly because of that difference in brightness. Roger Pickard of the BAA, my mentor, has shown me how to use other stars as comparison stars using Guide9.1 software that I have now ordered. Meanwhile he did the work to find two comparison stars that were similar in magnitude. I intend having another go at this with a longer time exposure to get increased accuracy. For this set of readings I was using 30s.