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ASNSW's Variable Star Section

Section Leader: Adrian Saw


The section assists new comers to variable star observing, in observing techniques, how to record your findings and what to do with them. The Society has several active members who can assist in this fascinating branch of astronomy. The recording of variable star estimates, gives your hobby of astronomy a real tangible connection to the professional astronomers who use your accumulated data in their researches.


Right back to basics! It’s a star whose brightness varies over a period of time. It may be only a few tenths of a magnitude or 10+ magnitudes. The time span may be recorded in parts of a day or 10’s of years. All stars are different, but on saying that most variables are cotagorised into classes depending on their magnitude changes (amplitude) and /or the time period these variations take place. They fall into categories such as Mira (long period variable), Cataclysmic, Recurrent Nova, RCB (which fade rather than brighten) and many other sub-classes. Rather than me plagiarizing all who have gone before me, there are many publications and Internet sites (listed below), which are of invaluable assistance and information to all variable star observers.

AAVSO Data for S Car


Most beginners are daunted by having to estimate the brightness of a star and think it is beyond them, in fact it’s fairly easy. The vast majority of known variable stars have comparison charts available for them. They can be obtained free of charge (in most cases) from organisations such as the AAVSO (American Association of Variable Star Observers) or RASNZ (Royal Astronomical Society of New Zealand) Variable Star Section.

These charts are produced of the localized area of the target star and where possible show many other stars of known magnitude as a comparison. Both the above-mentioned organisations have very useful beginner sections.

You can start with some of the brighter stars within the range of binoculars if you wish; this generally ensures you are observing the right star. Finding the fainter variables can be a little difficult at first some times as the charts may have faint stars on them which may be below your scope’s limiting magnitude.

  • When matching the star patterns from the charts to the one in the eyepiece you can be thrown off by your target star being visible or invisible, depending on where it is in its cycle of brightness.
  • You will have difficulty in estimating red stars as the longer you look at them the brighter they appear.
  • You may think the estimating of brightness against a comparison star is difficult.
  • When coming to send off your first observations to the AAVSO or the RASNZ you will worry that they are not correct.

All the above points may seem to be very daunting ones, believe me they are not problems at all. The tricks are easily learnt very quickly, you mentally discard your target star until you are sure of your field of view. Red stars you take several short glimpses at.

Methods of estimating can be using the step method, best done if you have a comparison stars brighter and fainter, and estimate where it falls between them. You can also estimate brightness by defocusing (outside focus is best) this is difficult to explain but easy to do at the eyepiece.

Your observations are part of a worldwide amalgamation of observations, which are averaged, your estimate may be a bit bright but someone else’s will be a bit faint. So the final outcome is somewhere about right.

There are many organised observing programmes which you may wish to join or do your own thing and observe a particular type of star, which ever suits you.

Some observers do nothing else than variable observation others may only keep an eye on maybe half a dozen stars on a regular basis. Which ever suits your observing programme?

If you do not wish to send your observation off directly to one of the variable star organisations the ASNSW can act as a central collator and send them off as a block


I have been a member of the society for about 20 years and observing variables for 15 years (is there anything else in the sky)? At the time I was a bit matter of fact about general deep sky observing and was then inspired by a talk given by another member (Surjit Wadhwa). I am a keen observer and reasonably experienced, but do not claim to be a variable expert. Hopefully I am able to direct others to find the right information.


Many other useful sites can be accessed through the AAVSO link page.


  • Observing Variable Stars by David H Levy
  • Variable Stars by Hoffmeister/Richter/Wenzel

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