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Light Curves update : 2015 November

Here is a selection of light curves that include observations received up to late November.
 
These are based on observations submitted by Matthew Barrett, David Scanlan, Graham Taylor, Richard Livingstone and Tony Markham.
 
If you have any observations from this period that you have not yet submitted, these would still be very welcome. You can email them to  variablestar@popastro.com
 
T Cephei
 
The light curve below shows nearly two cycles of variation of this Mira type variable.

As can be seen, the two maxima were not equal in brightness - this 'inconsistency' is quite normal behaviour for Mira type variables, and is part of their appeal to observers. Another unusual feature was that the 'pause' on the way to maximum that normally lasts 2-3 weeks did in early 2015 last for 2 months!

 

U Cephei 

The accompanying light curve shows the primary eclipse of U Cephei in 2015, based on observations reported up to the end of November.

The phases were calculated using the elements for U Cephei in the General Catalogue of Variable Stars (GCVS).

If the primary eclipse was occurring on schedule, then it would be centred on phase 0 in this light curve.

Clearly the primary eclipse is occurring late, and is close to (calculated) phase 0.09. This corresponds to the eclipses occurring about 5 hours later than predicted by the GCVS elements.

Fortunately, for prediction purposes, more up-to-date elements for U Cephei are available from the University of Krakow and it is these (SAC) elements that are used when publishing predictions in Popular Astronomy  for eclipses of U Cephei.

 

AF Cygni

The accompanying light curve shows observations of this semi-regular variable made so far in 2015.

With AF Cygni being a semi-regular variable, the brightness changes don't repeat exactly from one cycle to the next, but there is a degree of periodicity in the changes.

The period of AF Cygni is (usually) about 3 months. In 2014, however, it appeared to be varying with a period almost double this and the brightness variations of early 2015 suggested that this was still the case.

However, in the second half of 2015, the 3 month variations seem to have been re-establishing themselves.

 

R Leonis

The Mira type variable R Leonis is located only a few degrees away from the bright star Regulus and is the only variable star in Leo bright enough to observe (for part of each year) using binoculars.

The 2015 peak of R Leonis was somewhat unfavourable, occurring just as the star was emerging from the September morning twilight. Hence the rise to maximum was unseen, but early risers could monitor the subsequent fade.

The observations suggest that this was a fairly average maximum of R Leonis.

 

The next maximum will be even more inconveniently placed, occurring in July 2016, when Leo is near conjunction with the Sun.

 

RR Lyrae 

This light curve shows all of the observations of RR Lyrae made up to November 2015.

These have been combined to show one cycle of variation.

The period of RR Lyrae is approx 13 hours and the light curve almost repeats from one cycle to the next.

However, small differences can occur between each cycle of variation and hence the "scatter" in the observations may in part be due to these changes, rather than simply being wholly due to the scatter that occurs in visual observations that have been made under a variety of observing conditions.

 

Beta Lyrae

Two light curves are shown here. The phases calculated for the one on the left were based on the (old) GCVS elements for the star. The phases in the one on the right are based on the (newer) SAC elements for the star. If eclipses were occurring on schedule, then the (deeper) primary eclipse would be centred on phase 0.

 

 

 

 

 

 

 

 

As can be seen, the eclipses of beta Lyrae are well out of step with the GCVS elements but are fairly well in step with the SAC elements. It is these latter elements that are used when predictions are published in Popular Astronomy.

 

R Ursae Majoris

The November 2015 maximum of R UMa seems to have been somewhat fainter than normal, but has also been quite broad.

Maxima of R UMa are usually brighter than this - mag 7.2 being the average - but also tend to be sharper.

 

it is likely that, for observers with reasonably dark observing sites, R UMa will remain visible in binoculars well into December.

 

The next maximum of R UMa is predicted to occur in September 2016, which would suggest that it will become bright enough to be seen in binoculars during August.

 

 

Z Ursae Majoris 

Z UMa has produced two unusually faint minima in 2015, with one occurring in April and the second in early November.

Most variable star listings quote a magnitude range of 6.6-9.3 for Z UMa, but both 2015 minima seem to have dropped below this lower "limit", with some observations reported elsewhere quoting magnitudes as faint as 9.9.

The latest observations show that Z UMa is now brightening again and we can look forward to it reaching maximum in the late winter.

 

AG Pegasi 

Although not part of the SPA VSS "programme", it is interesting to see how AG Peg has behaved during 2015 - it has had a rather interesting year!

Prior to 2015, AG Pegasi was best know for an outburst in brightness in the late 19th century that took it up to around magnitude 6.0. However, the outburst was only recognised retrospectively several decades later and our knowledge of that outburst is rather patchy, being based on images on a rather sparse collection of photographic images.

In recent decades, AG Peg had been close to magnitude 9.0, showing fluctuations of a few teneths of a magnitude.

In mid June 2015, however, AG Peg brightened significantly. The accompanying light curve shows observations made by Tony Markham.

The peak brightness at the start of July was somewhat lower than that seen in the 19th century - but for that earlier outburst it took more than a decade from the start of the outburst to the peak being reached. It may be that the current changes are just a precursor to a larger outburst that it still to come.

Added by:  Tracie Heywood