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Variable Star light curves for 2016

Here are the light curves for 2016 for the variable stars that are on the section's programme. Also included at the end are light curves for a number of interesting variable stars that have been monitored by section members duriing 2016.

The following people have submitted variable star observations during 2016:  Matthew Barrett, Tracie Heywood, Don Matthews, Tony Markham, Jonathan Shanklin, Brendan Shaw, Bob Steele and Graham Taylor.

Special mention should go to the numerous DSLR observations made through a telescope by Don Matthews. These covered Mira type variables and R CrB while they were below the binocular visibility range and have filled gaps that would have otherwise occurred.

 

R Bootis

The Mira type variable R Bootis produced two maxima during 2016.

The first, during the early spring, occurred with Bootes well placed for observation in the evening sky.

The second occured in late November, with Boortes sinking into the evening twilight, leaving R Bootis effectively only observable in the pre-dawn sky.

Both maxima seem to have peaked just above mag 7.0.

In between, R Boo faded into a late summer minimum at around mag 12.2.

The next maximum of R Bootis is due to occur in mid July 2017.

 

RW Bootis

This semi-regular variable had been showing a larger than usual amplitude during 2014 and 2015 and in 2016 the amplitude seems to have increased further.

In early 2016, RW Bootis was fading and dipped all the way down to about mag 9.2 during the spring.

It then brightened again, reaching mag 7.8 by the early autumn, after which it started to fade once more.

Once again,there has been no sign of the 209-day period that it usually quoted for RW Bootis. The brightness variations in recent years have instead suggested a period of 10-11 months.

 

RZ Cassiopeiae

As is often needs to be the case given the UK weather, this light curve brings together observations made of parts of several different primary eclipses observed during 2016. Having said that, Bob Steele did manage to observe nearly all of the eclipse that took place during the evening of Dec 28th.

The horizontal axis shows the (predicted) phase (i.e. fraction of orbit completed) of RZ Cas at the time that each observation was made. These were calculated using the orbital elements listed in the General Catalogue of Variable Stars (GCVS). It is the standard when analysing eclipse measurements to measure any discrepancies in observed eclipse timings relative to the GCVS predictions.

If eclipses were occurring in line with the GCVS predictions, the primary eclipse would be centred on (predicted) phase 0.

As can be seen, however, the point of mid eclipse is shifted to the right and is near phase 0.065. Given the orbital period of RZ Cas of 1.195247 days, this corresponds to the eclipse occurring approx 110 minutes later than predicted by the GCVS.

Fortunately, when it comes to predicting future eclipses, more recently measured orbital elements are available. These elements, which are used for the predictions issued by the section, quote a slightly longer orbital period of 1.1952524 days - a tiny difference, but it all adds up over hundreds of orbits.

 

T Cephei

This Mira type variable has shown rather prominent 'pauses' during its rise to maximum in recent years.

Although short pauses near mag 8.0 are not unusual, the pauses in 2015 and 2016 were unusually prolonged, with the 2015 pause lasting for nearly 3 months.

Despite the pause, the 2016 maximum was not delayed. Indeed, it probably occurred slightly earlier than predicted.

Follow its May peak, T Cep faded and reached minimum, at about mag 10.2 in the early autumn. By the end of the year, it was just approaching the brightness at which the next 'pause' would occur.

The next maximum of T Cephei is due to occur in June 2017.

 

U Cephei

As was the case for RZ Cas, the UK weather prevented any complete primary eclipses of U cephei being observed during 2016 and it has been necessary to combine observations of several partly observed eclipses in order to create this light curve. This has resulted in some scatter (due to differences in observing conditions), but the general shape of the light curve can still be seen.

Also, as for RZ Cas, the horizontal axis shows the phase, as predicted by the orbital elements in the GCVS, at the time of each observation.

Again, as was seen for RZ Cas, the eclipses not centred on (GCVS predicted) phase 0, but is somewhat later - near 0.095. Taking into account the 2.4930475 day period listed for U Cephei, this corresponds to the primary eclipses occurring more than 5 hours later than predicted by the GCVS.

And, once again as for RZ Cas, the predictions published by the section are much more accurate because they are based on the most up to date orbital elements for U Cephei.

 

Delta Cephei

With the period of Delta Cephei being just 5.37 days, it isn't easy to get enough observations during any 5.37-day cycle to generate a good light curve.

However, since the brightness changes repeat exactly from one cycle to the next, it is possible to combine all observations of Delta Cephei made during a year into  single light curve covering a single cycle (or, as in this example, so as to make the shape of the light curve clearer, 1.4 cycles).

Although there is some scatter due to differences between observers and also due to variable sky conditions (moonlight, light pollution, etc) for different nights/locations, the general shape of the light curve can be seen, with Delta Cephei being brightest at phase 0 and then fading slowly to be faintest at around phase 0.7. This is then followed by a steeper rise back to maximum.

 

Omicron Ceti (Mira)

We almost saw Mira at maximum in 2016.

Maxima in recent years have been occurring during the interval from late March to mid July when Mira is close to conjunction with the Sun and thus unobservable from the UK.

The 2016 maximum was due to occur in early April. However, the final pre-maximum brightness estimate in mid March (just before Mira was lost in the evening twilight) was at mag 3.3, suggesting that the maximum might be occurring earlier than predicted or was set to be brighter than average.

By the time that Mira emerged from the morning twilight in late July, it was well on the way down to its mid autumn minimum. By November, Mira was brightening again, on the way to its late February 2017 maximum.

 

R Coronae Borealis

Having entered a deep fade in June 2015, R CrB had spent the second half of 2015 and the first half of 2016 near minimum.

In the second half of 2016, however, R CrB started to slowly brighten.

Initially, this was at a rate of around a magnitude per month. In the early autumn, it speeded up, the paused in early November beefore resuming once more. By the end of 2016, R CrB was at about mag 7.9 - still nearly two magnitudes short of the maximum level we used to see before 2007.

We wait to see whether R CrB will brighten all the way back to maximum or whether, as in early 2015, it will fall short and fade again.

 

U Coronae Borealis

It can be quite a challenge to get a good light curve of an eclipse of U CrB.

When U CrB is best placed for observation between April and August, the nights are much shorter than U CrB's 11 hour eclipse.

When the nights are long during the autumn and winter, U CrB is poorly placed for observation.

Hence it is always necessary to combine observations made during several different eclipses and hope that these together produce a light curve that covers all of the eclipse.

As can be seen in the accompanying light curve, U CrB was observed several times during its fade into eclipse in 2016 ... but only once during the rise from eclipse.

It does look, however, like minimum brightness occurs near (GCVS predicted) phase 0.04. This corresponds to eclipses of U CrB occurring more than 3 hours later than predicted by the GCVS elements.

 

AF Cygni

This semi-regular variable is listed as having a period of 92 days - approx 3 months. However, as the accompanying light curve shows, this isn't always obvious in its brightness variations.

The light curve shows four obvious maxima, at intervals of 6-7 months. In some cases, however, there are less bright intermediate maxima.

In 2016, for example, There are bright maxima of AF Cygni in May and late November, but there also appears to be an intermediate, less bright, maximum in August.

Thus, if we take into account these intermediate fainter maxima, there are actually some signs of the 3-month period.

 

Chi Cygni

In contrast to the bright maxima of May 2013 and August 2015 and the unusually faint maximum of June 2014, Chi Cygni produced a rather "normal" maximum in 2016.

It reached binocular visibility during July and after a steep rise in brightness during August, produced a fairly "flat" maximum, at around mag 5.0, during September, before fading again.

By the end of the year, Chi Cygni was about to drop below binocular visibility.

The next peak is due to occur in late October 2017.

 

Zeta Geminorum

Although Zeta Geminorum, like Delta Cephei, is a Cepheid variable, its light curve has a somewhat different shape.

The light curve of Delta Cephei shows a steep rise in brightness followed by a slower fade ; that of Zeta Gem is symmetrical and more or less sinusoidal, with peak brightness occurring at phase 0 and minimum brightness at phase 0.5.

This is fairly well seen in the accompanying light curve.

The reason that the light curves have different shapes is because the two stars pulsate in different ways.

 

Alpha Herculis

As in 2015, magnitude values have been omitted from the vertical axis of this light curve - the scale of the axis amounts to one magnitude.

The reason for omitting the magnitude values is that with Alpha Her being a red star, systematic differences can occur between observers. In some cases this amounted to 0.4 mag.

Had this not been adjusted for when plotting the light curve, the resulting "scatter" would have dwarfed the observer brightness changes.

Whereas 2014 and 2015 saw a gradual rise in the average brightness, the average brightness seems to have changed little during 2016. minima are apparent in late February and early September - approx 200 days apart. This is midway between the posssible 100-day and 300-days periods sometimes suggested for Alpha Her. There is however a (less convincing) possible minimum midway between them in early June.

 

u Herculis

This eclipsing variable, also known as 68 Herculis, can be a tricky star to observe. It is a bit on the faint side for naked eye observation (given typical levels of light pollution in the UK) but also a bit bright to observe using binoculars.

This can lead to differences of several tenths of a magnitude between estimates reported by different observers ... potentially leading to a rather messy light curve.

The accompanying light curve has been generated after attempting to allow for these differences between observers.

The phases have been generated using the latest Krakow elements, so we would expect to see the (deeper) primary eclipse centred near phase 0 and the (shallower) secondary eclipse centred near phase 0.5. Although there is a fair amount of scatter in the light curve, this does indeed appear to be the case.

 

R Leonis

The 2016 maximum of the Mira type variable R Leonis was not conveniently timed for observers.

In 2015, R Leonis had been just past maximum when it emerged from the morning twilight during September.

By the time that R Leonis became visible in the evening sky in early 2016, it was getting close to minimum.

During the late spring, it was starting to brighten towards its next maximum, but was lost in the evening twilight in early June, more than a month before maximum was due.

By the time that R Leo had emerged from the morning twilight in autumn 2016, it was well past maximum and closing in on its next minimum.

The next maximum of R Leonis will be (a bit) more conveniently timed - in early June 2017, although this is only just before it sinks into the evening twilight.

 

Beta Lyrae

The accompanying light curve shows all observations of beta Lyrae reported during 2016 combined into a single light curve showing1.5 cycles of variation.

The phases were calculated using the GCVS elements. However, these are quite old now (dating back to the late 19th century) and the period of beta Lyrae has lengthened significantly since then.

Hence, whereas the deeper primary eclipse would be expected to be near (predicted) phase 0, this light curve has it close to phase 0.35. Indeed, the discrepancy is a lot more than that and, over many decades, has built up - and now equates to several orbital periods, plis 0.35 !

 

RR Lyrae

Despite the number of observations of this star being somewhat lower than in 2015, the general shape of the light curve can still be seen.

Like is seen in Cephei variables, such as Delta Cephei, there is a steep rise to maximum brightness followed by a slower fade back to minimum.

However, whereas Cepheid variables are very predictable, the observed times of maxima of RR Lyrae type variables can drift away from those predicted by the elements in the GCVS. This light curve show peak brightness occurring just before phase 0.4, corresponding to a discrepancy of around 5 hours.

 

Alpha Orionis (Betelgeuse)

Betelgeuse is a semi-regular variable whose brightness changes are generally rather slow.

A period of 2070 days (approx 6 years) is often quoted but observations over recent decades would better fit in with a period of around 1600 days (4.5 years).

During 2016, Betelgeuse seems to have continued with the slow brightening seen during 2014 and 2015.

 

U Orionis

The maxima of this Mira type variable are becoming increasingly harder to observe. The average interval between maxima is about a week longer than a year. Maxima are currently occurring in April, but are getting progressively closer to when U Orionis disappears into the evening twilight during May.

As can be seen in this light curve, we did manage to follow U Ori all of the way to maximum in 2016, but the start of the fade was not seen. Observations late in the year found U Ori to be near minimum.

Peak brightness seems to have been just below mag 6.0, making this a slightly brighter than average peak.

The next maximum is due to occur in late April 2017.

 

Beta Persei (Algol)

Although often promoted as a good target for beginners, capturing a good light curve of an eclipse of Algol is quite tricky. Many of its comparison stars are located some distance in the sky from Algol. Factors such as haze and light pollution can often make it difficult to accurately compare star brightnesses, especially when they or Algol are fairly low in the sky. In addition, Algol is only well placed for observation from September to February. At other times of the year, though visible, it is relatively low in the sky.

And even when Algol is well placed for observation, there is a high chance that cloud will intervene before you have seen all of the eclipse!  Hence it is usually necessary to combine observations made of parts of different eclipses and this tends to increase the scatter in the light curve.

As can be seen in the accompanying light curve, most observations made in 2016 were during the fade and just as Algol was reaching its minimum. It is unclear from the light curve as to when the minimum ended and Algol started to brighten again.

However, it is clear that mid eclipse was shifted away from (GCVS predicted) phase 0, and was probably near phase 0.05, corresponding to eclipses occurring around 3 hours late.

As for the other Eclipsing Binaries mentioned earlier, the eclipse predictions for Algol published in the VSS News section and in Popular Astronomy use more up-to-date orbital elements and so should be accurate.

 

R Scuti

Despite its reputation for producing alternate deep and shallow minima, R Scuti in 2016 seems to have produced one deep minimum (April), followed by three shallower minima (June, September, November).

Compared with recent years, even the deep minimum of 2016 was relatively shallow (as described in  this report ) - the years 2013 and 2014 had both produced deep minima that dropped to asround or below mag 8.0.

The long term light curve for R Scuti does show years in which the depth of the minima can be relatively shallow.

There is, however, there is no clear pattern in this longer term activity that would help us foresee how R Scuti might behave during 2017.

 

R Serpentis

The average period of this Mira type variable is approx 357 days, so peak brightness is occurring about a week earlier each year.

Currently, maxima are occurring during July when Serpens is well placed in the night sky. This is good.

On the downside, the June weather can often let us down with many of the short nights proving cloudy. This happened once again in 2016 and, as a result, the final stages of the rise to maximum were not well seen.

R Ser produced a rather flat maximum, staying at around mag 7.3 (slightly fainter than the average peak) throughout most of July. A steady fade then set in and by October it was only visible in telescopes.

The next peak of R Ser is due to occur in July 2017.

 

R Trianguli

Some maxima of R Trianguli are better placed for observation thn are others.

The 2015 June peak occurred with R Tri poorly placed in the pre-dawn sky and few observations were made.

The rise to the Marcgh 2016 was well observed but, soon after starting to fade, R Tri then sank into the evening twilight.

The December 2016 was more ideally timed and turned out to be a brighter than average one. From mag 11.2 in late July, R Tri brightened to reach around mag 5.7 in late November, slightly earlier than predicted.

The next peak of R Tri is due to occur at around the start of September 2017.

 

R Ursae Majoris

The Mira type variable R UMa started 2016 fading from its late 2015 peak.

Minimum brightness appears to have occurred during April.

Maximum was due in early September. This may have occurred slightly early (in late August), although with the maximum being rather flat, it is difficult to be certain.

Peak brightness appears to have been close to mag 7.0.

By the end of the year, R UMa had dropped below binocular visibility.

The next maximum is due to occur in early July 2017.

 

S Ursae Majoris

S UMa, another Mira type variable, started 2016 as a telescopic object, but brightened to become visible in binoculars by the mid spring.

Peak brightness, near mag 8.3, occurred during June, followed by a fade down to minimum, which occurred near mag 12.3 in the early autumn.

S UMa then brightened again and by the end of 2016 was close to maximum again. Once again, peak brightness  seems to have been near mag 8.5.

Another maximum is due to occur in August 2017.

 

T Ursae Majoris

At the start of 2016, this Mira type variable was approaching maximum. This occurred during January, at around mag 8.3.

T UMa then faded, reaching minimum brightness, near mag 13.0, in the late spring.

The early autumn maximum of T UMa occurred with Ursa Major less well placed for observation, but it seems to have been similar in brightness to the January maximum.

By the end of the year, T UMa was well on the way down to its next minimum.

The next maximum of T UMa is due to occur in June 2017.

 

Z Ursae Majoris

The semi-regular variable Z UMa is one of the more reliable stars of its type, regularly covering most of its brightness range. Indeed, in recent years, it has been dropping below its former lower limit (of mag 9.3) during minima.

The brightness variations don't repeat from one cycle to the next.Instead, there appears to be two periods operating alongside each other. Both are close to 6 months in duration, but their combined effect is to cause the shape of the light curve to change slightly from cycle to cycle. Currently, this is producing "flat" maxima, probably with small dips part way through and sharper minima.

Most of Z UMa's brightness changes during 2016 are well covered in the light curve. It is unfortunate, however, that a cloudy June  hindered the monitoring of the the mid-year sharp rise in brightness from minimum to maximum.

 

Observations of other stars

Many observations were also reported of other variable stars. Here are light curves for some of them:

 

Eta Aquilae

This Cepheid variable can be tricky to observe from the UK as for much of the year it is only observable when farly low on the sky. At such times, haze can make it difficult to estimate its brightness. It is only during the summer and early autumn that Eta Aquilae is better placed.

This light curve is based on observations by Matthew Barrett, Bob Steele and Graham Taylor during 2015 and 2016.

Despite the challenges, the light curve clearly shows the general shape of its light curve, with a steep rise in brightness, near phase 0, is followed by a slower fade back to minimum, near phase 0.8.

 

V Bootis

This semi-regular variable showed an amplitude of around 4 magnitudes during the first half of the 20th century, but had become less active in recent decades.

Recent years have seen the amplitude increasing again.

This light curve is based on observations by Jonathan Shanklin, Tracie Heywood and Tony Markham.

As can be seen, the amplitude during 2016 was around 2 magnitudes - the best seen for some time.

V Bootis is located near the star Gamma Bootis. For most of the time, observation of V Boo requires large binoculars or a small telescope.

The BAA VSS finder chart for V Bootis can be found  here

 

V Canum Venaticorum

This is a semi-regular variable star that is almost as rewarding to observe as Z UMa. It has a main period of variation of 192 days (approx 6.5 months), but additional periodicities can cause the amplitude and shape of the light curve to change fom one cycle to the next.

The accompanying light curve is based on observations made during 2016 by Jonathan Shanklin, Tracie Heywood and Tony Markham.

As can be seen , the 192-day period was not particularly apparent. Indeed, V CVn appeared to have already gone through two cycles by the end of May.  Looking back at recent years, however, there is probably a reason for this. In 2014 and 2015, V CVn was showing a 'hump' on the rise to maximum. The narrow spring 'maximum' seen in 2016 was most likely a very pronounced version of this 'hump' !

The mid-year maximum appears to have been slightly broader, however, and the gap between this and the maximum being approached at the end of the year does look more in line with a 192-day period.

The BAA VSS finder chart for V CVn can be found  here

 

T Coronae Borealis

This recurrent nova, which produced outbursts to mag 2 in 1866 and to mag 3 in 1946, was observed by Don Matthews and Jonathan Shanklin.

This light curve may not, at first sight, seem particularly dramatic - except that T CrB, though still near minimum, has during 2015 and 2016 been brighter than usual by around a magnitude.

Whether this is a sign that the next outburst is imminent is unclear ... but T CrB did brighten by a similar amount during the years leading up to the 1946 outburst.

Large binoculars or a small telescope are currently required in order to observe T CrB. If you don't possess these, you could alternatively just watch out for an extra star appearing near Epsilon CrB.

The BAA VSS finder chart for T CrB can be found  here

 

CH Cygni

This 'symbiotic' variable contains one star which is a semi-regular variable and one star which is an eruptive variable.

Sometimes, the semi-regular variables dominate the light curve ; at other times, the eruptive variations dominate. In the early 1980s, CH Cygni became as bright as mag 5.4 for a while ; in the mid 1990s, it briefly dipped as faint as mag 10.5.

2016 saw CH Cygni fade from being an easy to see binocular object to being one which requires larger binoculars or a small telescope.

What will happen during 2017 is not clear.

You can find the BAA VSS finder charts for CH Cygni  here .  Note that there is a mag 9.4 comparison star close to CH Cygni. When CH Cyg becomes faint, be sure that you are seeing CH Cyg separately from this star.

 

U Monocerotis

U Mon is a similar type of variable to R Scuti

This light curve shows the brightness variations of U Mon during the first half of 2016, as recorded by Jonathan Shanklin and Tony Markham.

The average period of U Mon is approx 92 days - just a little shorter than the length of this light curve.

We can see a deep minimum down to mag 7.2 in February and a (only slightly less deep) shallow minimum down to mag 6.8 in late March.

It is somewhat unfortunate that U Mon lies in a relatively bland area of sky (between Sirius and Procyon) and this makes it challenging to track down when star-hopping to it using binoculars. Otherwise, it would have been a strong candidate for inclusion on the section's programme.

The BAA VSS finder chart for U Mon can be found  here

 

V Vulpeculae

V Vul is also a similar type of variable star to R Scuti, but observation of it requires large binoculars of a small telescope. It has a period of approx 76 days.

This light curve shows observations made by Jonathan Shanklin during the second half of 2016.

Two minima are apparent - in late August and in mid November.

The BAA VSS finder chart for V Vul can be found  here

Once again, this is a star that is in a relatively bland area of sky, so locating it using binoculars can be a challenge. The star Zeta Cygni is probably the best starting point when 'star-hopping'.

Added by:  Tracie Heywood