Here is the annual variable star section report for 2017, summarising the observations made by SPA VSS members during the year.
Observations were submitted by Matthew Barrett, Don Matthews, Jonathan Shanklin, Bob Steele, David Smith and Tracie Heywood. Once again, we have Don to thank for following the Mira type variables through their telescopic minima.
This Mira type variable had been at maximum in late November 2016 and was fading from this peak during the early part of 2017.
Having passed through minimum, at around mag 12.7, in early April, R Boo then brightened again. The subsequent maximum probably occurred in mid July but, as the accompanying light curve shows, a short gap in observations close to the peak means that it wasn’t possible to say how bright the peak was exactly, but the indications are the it was close to the long-term average of mag 7.2. By the time that it was becoming poorly placed in the October evening twilight, R Boo had faded to 11th magnitude.
The next maximum of R Bootis is predicted to occur during February 2018.
There has been a significant change in the light curve of the semi-regular variable RW Bootis over the past 18 months.
The most noticeable change is that the amplitude has halved. The brightness at maximum is unchanged, but the minima are now much less deeper.
Closer inspection, however, shows that the maxima are now closer together – every six months rather than every 10-11 months.
Despite this change, the brightness variations of RW Bootis continue to be out of step with the catalogued period of 209 days (approx 7 months).
The accompanying light curve shows the observations of the primary eclipse of RZ Cas made during 2017.
These have been combined into a single light curve showing the phase (the fraction of the orbital period completed) at the time of each observation.
The phases were calculated using the “elements” for RZ Cas held in the General Catalogue of Variable Stars (GCVS).
If the primary eclipse was occurring in line with these elements, it would be centred at phase 0.
Clearly, it is shifted to the right – the shift to around (predicted) phase 0.07 corresponding to the eclipses occurring around 40 minutes later than predicted by the GCVS (the orbital period of RZ Cas being approx 29 hours).
Discrepancies such as this are quite normal for most short period eclipsing binaries and are due to their orbital periods gradually changing due to mass transfer between the two stars and/or the gravitational effect of a third (unseen) star in the system.
As can be seen from this five year light curve, the late spring 2017 maximum of the Mira type variable T Cephei was fainter than those of recent years. Following this rather flat-topped maximum, a quite steep fade set in and minimum brightness was reached during October. By the end of the year T Cep had brightened to 8th magnitude.
The ‘pause’ near mag 8.0 during the rise to maximum was once again a prominent feature, although slightly less so than in 2016.
It might seem from the above light curve that the scatter in the observations has increased over the years. This is, however, merely a consequence of there being more observers in recent years, the 2013-2014 section being based on the brightness estimates of a single observer.
The next maximum is predicted to occur in June 2018.
The accompanying light curve shows the observations of the primary eclipse of U Cep made during 2017.
As in the case of RZ Cas, these have been combined into a single light curve with the phases on the horizontal axis calculated using the elements in the GCVS.
Once again, the centre of the primary eclipse is clearly shifted to the right from (predicted) phase 0. The shift to around phase 0.09 corresponds to the eclipses occurring around 5 hours later than predicted by the GCVS (the orbital period of U Cep being approx 2.5 days).
U Cep can be followed throughout its eclipses using large binoculars (i.e. 70mm objective or larger). More common binocular sizes (such as 10×50) may struggle near mid eclipse unless sturdily mounted and/or from locations with good dark skies.
Eclipse predictions published in Popular Astronomy and elsewhere do not, of course use the GCVS elements. They instead base their predictions on elements derived from more recent observations.
This photometric light curve for the eclipsing variable EG Cephei was submitted by David Smith
With EG Cep being circumpolar, it should be possible to see the whole 13-hour cycle of brightness changes during a single winter night. The late winter weather was uncooperative, however, and David needed to make observations on a number of different nights spread over six weeks in early 2017 in order to cover all stages in the 13-hour cycle.
David’s reward for his efforts was this very impressive light curve. Since David was observing photometrically with a V filter he was able to clearly see the 0.3 magnitude dip at secondary eclipse as well as the deeper 0.9 magnitude primary eclipse and the slower changes in brightness between eclipses.
The accompanying light curve for shows the observations of the Cepheid variable delta Cep made during 2017, all combined into a single light curve.
The horizontal axis shows the phase – the fraction of its 5.37 day cycle completed at the time of each observation.
The first half of the light curve has been repeated on the right so as to make the shape of the light curve more obvious.
As can be seen, there is a steady fade in brightness from phase 0 down to around phase 0.7, followed by a steeper rise back to maximum.
omicron Ceti (Mira)
Having been unobservable for several years due to occurring close to solar conjunction, maxima of Mira have started to become better placed. The 2017 maximum was predicted to occur in late February.
A steep rise in brightness was observed during January, but poor weather hindered observations of the maximum unfortunately. The indications are, however, that it was a fairly normal maximum.
By the time that Mira emerged from the morning twilight during the late summer it was approaching its 9th magnitude minimum.
The rise in brightness from Mira’s early autumn minimum was slow initially but speeded up during November and December, reaching mag 4.4 by the last week of December. Maximum is predicted for late January.
R Coronae Borealis
It used to be the case that R CrB would spend most of its time at maximum, near mag 6.0, and observers would monitor it to try to catch the start of one of its sudden dramatic fades.
This hasn’t been the case for more than a decade now. More than 10 years on from its big fade of mid 2007, R CrB still hasn’t returned back to maximum and has spent most of this interval down around magnitude 14-15.
In recent years R CrB has “made attempts” to return back to maximum, reaching around mag 7.0 in early 2015 before fading again and, most recently, again reaching binocular visibility in autumn 2016. This latest recovery seemed to have “run out of steam” while still more than a magnitude below its “normal” maximum level. This time, as the light curve shows, rather than plummet back down again, it has continued to edge up in brightness very slowly and was around mag 6.7 by the end of 2017.
U Coronae Borealis
As in previous years, the number of observations made of the eclipsing variable U CrB has been rather small.
As the light curve shows, only three of these were during the primary eclipse and all seem to have been during the emergence from eclipse.
The limited number of observations is probably linked to the 3.45 day period of U CrB being longer than that of the other Algol-type eclipsing variables on the section’s programme. In addition, U CrB is best placed for observation during the spring months when the nights are rapidly becoming shorter. This can mean that there are long spells when no favourable eclipses occur during the hours of darkness.
The peak brightnesses of this Mira type variable have differred considerably in recent years. Those of 2013 and 2015 had been much brighter than average, while that of 2014 was one of the faintest ever. The early autumn maximum of 2016 had in contrast been fairly “average”. How would chi Cyg fare in 2017?
Observations by Don Matthews in late spring 2017 showed chi Cyg to be close to minimum during May and starting to brighten during June.
By mid summer, observations were suggesting that another very bright maximum of chi Cyg (predicted for late October) might be on the way. By late July, it was already mag 8.1 and by mid August had reached mag 6.2, little more than a magnitude short of its average maximum brightness. The brightening then slowed however, but by late September it was passing its average peak value, eventually reaching around mag 4.5 by the end of October – brighter than on average, but not as dramatically so as the summer observations might have suggested. A fade then set in and by the end of the year chi Cyg was down to around magnitude 7.0.
The next maximum of chi Cygni is due to occur in early December 2018.
The accompanying light curve for shows the observations of the Cepheid variable zeta Gem made during 2017, all combined into a single light curve.
The horizontal axis shows the phase – the fraction of its 10.15 day cycle completed at the time of each observation.
The first half of the light curve has been repeated on the right so as to make the shape of the light curve more obvious.
As can be seen, the light curve has a different shape from that of delta Cephei. Whereas in delta Cephei there is a steady fade in brightness from phase 0 down to around phase 0.7, followed by a steeper rise back to maximum, the light curve for Zeta Gem is more sinusoidal, with minimum brightness occurring near phase 0.5.
The accompanying light curve shows the brightness variations during the years 2014 to 2017.
Since some observers routinely see this red variable several tenths of a magnitude brighter than do other observers, the reported estimates have been adjusted to allow for this and therefore no magnitude values are plotted on the vertical axis, suffice to say that Alpha Her has been in the upper part of its catalogued brightness range during these years.
Although the observed brightness changes have been small, the indications are that there was a minimum in May, followed by a maximum in July and another minimum in September.
Overall, the trend during 2017 was for a slight fade, reversing the brightening trend of the earlier years.
The beta Lyrae type eclipsing variable falls in the awkward area between naked eye and binocular observations. From most observing sites it is too faint for the former, but it is also a little on the bright side for binocular observers.
This probably explains why the number of observations each year is relatively small and why there is quite a bit of scatter in the light curve.
Despite this, the main features of the light curve can be made out – a deep primary eclipse near phase 0 and a shallower secondary eclipse near phase 0.5, with brighter spells in between the eclipses.
The maxima in recent years of the Mira type variable R Leonis have not been conveniently timed.
The 2015 maximum occurred just before R Leonis emerged from the September morning twilight, while that of 2016 occurred with Leo near solar conjunction.
The 2017 maximum was a little better placed, scheduled to occur in early June, just as Leo was sinking into the evening twilight. This gave observers the opportunity to monitor its rise to maximum.
Minimum brightness occurred during the late autumn and by the end of the year R Leo had started the rise towards its 2018 maximum, which is predicted to occur in mid April.
RR Lyrae is a pulsating variable star whose cycle of brightness variations takes approx 13 hours. It is a similar type of star to the Cepheid variables, but the brightness variations do not quite repeat exactly from cycle to cycle. However, they almost do and so it is still possible to combine all of the observations from 2017 into a single light curve showing how the brightness changes during the cycle.
Unfortunately, by chance, there are few observations covering the early part of the cycle but the general shape of the light curve can still be made out. There is a steep rise in brightness to a maximum near phase 0.3, followed by a slower fade.
beta Lyrae is an eclipsing variable with an orbital period of approx 12.94 days. Both primary and secondary eclipses are readily visible and there is no period of constant brightness between eclipses.
The accompanying light curve shows the light curve for beta Lyrae during 2017, with all observations having been combined into a single light curve.
The horizontal axis shows the phase – the fraction of the orbital period completed at the time of each observation. With the orbital period of beta Lyrae having diverged greatly from that given in the GCVS, there is no point in plotting a light curve against the GCVS predicted phases. Hence, the phases shown here have been calculated using the more up to date “Krakow” elements.
Both eclipses are clearly seen – near phases 0 and 0.5. The secondary eclipse near phase 0.5 is actually really a little shallower than the primary eclipse, but a couple of “stray” observations (both by the Director – time to own up!) make this less obvious in the light curve.
This two year light curve for the Mira type variable U Ori illustrates how the annual maxima are currently only marginally visible. The rise to maximum during the early spring is well seen, but by the time that maximum is reached, U Ori is sinking into the evening twilight.
By the time that U Ori is observed again in the morning sky, it is approaching minimum and then starting to rise back towards its next maximum.
Although nether maximum was clearly seen, it is apparent that the 2017 maximum was around a magnitude fainter than that of 2016.
Alpha Orionis (Betelgeuse)
This semi-regular variable is only observable for about 8 months of the year and during two of these months estimating its brightness can be difficult due to significant differences in height above the horizon between Betelgeuse and its comparison stars.
Nevertheless, the general trends can still be seen in this light curve which covers the years 2014 to 2017.
The trend during 2017 seems to have been one of a slight fade, reversing the brightening trend of the previous three years.
This five year light curve for R Scuti shows how much the amplitude of R Scuti has reduced since its unusually deep and prolonged minima of 2013.
Such year to year differences are not unusual for R Scuti, however, It does, from time to time, have spells when the amplitude is large for several years, while during other spells any minima that occur are are rather shallow.
2017 seems to have been one of R Scuti’s “quieter” years. No particularly deep minima seem to have occurred, but shallow minima, dipping to only around mag 6.0, were seen in late May, mid August and early November.
R Scuti disappeared into the evening twilight during the third week of December, but starts to emerge from the morning twilight in mid January.
The accompanying light curve shows the brightness changes of the Mira type variable R Serpentis during 2016 and 2017.
R Serpentis has an average period of 357 days and thus reaches peak brightness about a week earlier each year. Maxima are currently occurring in early July.
As can be seen, the 2017 maximum was significantly brighter than that of 2016.
The next maximum is predicted for late June 2018.
The accompanying light curve shows the two most recent maxima of the Mira type variable R Trianguli.
The gap during the spring and early summer of 2017 is due to Triangulum being close to conjunction with the Sun.
Both maxima reached similar brightness levels, with the late summer 2017 being slightly the brighter. It is noticeable, however, that the two maxima have quite different shapes, with the 2017 peak being distinctly narrower.
R Trianguli ended 2017 near minimum brightness. During early 2018, with will be brightening, but the May 2018 maximum will be unobservable due to Triangulum being near solar conjunction.
R Ursae Majoris
The Mira type variable R Ursae Majoris has a period of approx 10 months.
Two maxima can be seen in the accompanying light curve. As is commonly the case for Mira type variables, the rises to maximum were steeper than the subsequent fades back down to minimum.
Both maxima were in line with the long-term average peak brightness of mag 7.2.
Following its early summer maximum, R UMa spent the later part of 2017 fading towards minimum, which occurred during December. The next maximum is predicted to occur in early May 2018.
S Ursae Majoris
This light curve for the Mira type variable S Ursae Majoris for 2016-2017 shows three maxima.
With S UMa being a particularly red star, there can be significant differences in the magnitudes reported by different observers. This can make it tricky to compare the relative brightnesses of the three peaks, although the indications are that they were largely similar.
A more distinctive difference can be seen for the minima, with the spring 2017 minimum being somewhat less faint than the minimum of autumn 2016.
The next maximum of S UMa is predicted for early April 2018.
T Ursae Majoris
The accompanying light curve shows the three most recent maxima of the Mira type variable T Ursae Majoris.
Cloudy skies hindered observations of the rise to maximum during spring 2017.
It can be seen, however, that the late spring 2017 maximum was considerably brighter than the previous two.
The subsequent fade took T UMa down to 13th magnitude for its autumn minimum, but by the end of the year it was brightening again.
The next maximum of T UMa is predicted for late February 2018, when Ursa Major will be climbing high in the evening sky.
Z Ursae Majoris
The light curve below shows the brightness changes in the semi-regular variable Z Ursae Majoris since the start of 2014.
There appear to be some clear trends in the light curve. From 2014 to spring 2016, the minima are becoming deeper, after which they become brighter again. The late summer 2017 maximum also appears to have been somewhat brighter than those in earlier years.