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Frequently Asked Questions

Who are the ASSA Variable Star Group?

 Well, for starters, none of us have any formal qualifications in astronomy or astrophysics. Most of us have day jobs (or shift work), and have learned our astronomy from books, from other people like us - and by spending thousands of hours observing the night sky.

None of us have a Perfect Observing Site or huge telescopes; in fact one of us does all his (numerous) observations with 11x80 binoculars from a very light-polluted suburb! Another one of us does over half of his observations with 7x50 binoculars.

About the only things we have in common are a particular interest in variable stars, a desire to contribute to astronomical research, and a dislike of cloudy nights.

Discoveries by Group members include (so far) two previously unknown variables; and a comet.

Why are amateur variable star observers more important than ever?

 Because amateur astronomers have more telescope time than professionals. The typical professional astronomer gets only a few hours of observing time per month, allocated in advance by telescope management committees. If the night happens to be cloudy then it's bad luck!

Therefore, professional astronomers can't monitor every variable star. They rely on dedicated amateurs to provide virtually all of the basic information about a variable's magnitude range, period, and visual characteristics.

So how can amateurs contribute? In at least two ways: monitoring the long-term behaviour of variables; and providing an "early warning" of sudden or unusual stellar activity. ASSA has a Variable Star Group to coordinate and assist amateurs who are interested in this work. Local tuition and assistance from experienced observers is available.

Where can I get some variable star charts?

 A few charts have been saved in GIF format and may be downloaded from the Variable Stars main page. We also have a complete collection of published VSS,RASNZ charts (~1200 variables!) and a limited selection of AAVSO charts.

Printed copies of all the charts in our collection can be purchased from ASSA at a cost of $A0.50 per copy. Send us the names of the stars you want charts for, your postal address, and payment. We accept bank cheques from an Australian bank, money orders from post offices everywhere, or Australian cash, preferably unmarked....

If you use binoculars, my booklet Binocular Variables for Southern Observers contains charts for over 100 bright variables.

Many variables in the Magellanic Clouds can be observed with the aid of Morel's Visual Atlases.

If you are observing from the Northern Hemisphere; we suggest you contact the AAVSO or the BAA variable star section for suitable charts. Although we do observe some equatorial and slightly northern stars, most of our charts are for stars in the More Scenic Half Of The Sky!

Classification codes

 Are you baffled by GCAS, CEP(B), EA+ZAND and those other pieces of alphabet soup that appear in variable star lists? Read "How To Decipher Those Classification Codes" and learn what the codes mean in non-technical language!

How Many Variable Stars are There?

 Nobody knows. Seriously! Defining a star as variable requires at least two reliable magnitude measurements, at two different times, which disagree with each other. How much of a discrepancy qualifies as a "real variation" is a matter of opinion. Anything below 0.2 magnitudes is virtually undetectable to the human eye; and visual observers trying to follow a star with less than 0.4 magnitude variation are wasting their time. They should use a CCD or photometer instead.

As of mid-1998, the number of proven and suspected variables in our Galaxy was about 60,000. The statistics are not good. Only 5% of the 300 billion stars in our Galaxy are directly visible, many starfields have not been thoroughly checked, and certain types of variables are hard to detect during astronomical surveys....

Variables have been found in other galaxies too. For example, Volume 5 of the General Catalogue of Variable Stars (4th edition) lists 4801 known variables in the Large Magellanic Cloud - a number expected to increase enormously in the aftermath of the MACHO Project.

How Many Species of Variable Stars are Known?

 No two variables are exactly the same, but seven major categories are recognised:
  • Eruptive
    variation caused by flares or shell ejection; eg: flare stars, T Tau variables, RCBs, S Doradus stars.

  • Pulsating
    radial or non-radial pulsation eg: Miras and semiregular variables, Cepheids, RV Tau stars.

  • Rotating
    variation caused by starspots, magnetism, or changing shape. eg: pulsars, elliptical stars and magnetic variables.

  • Cataclysmic
    variation caused by explosions of the star or an accretion disc. Includes dwarf novae, classic novae & supernovae.

  • Eclipsing
    binaries where one component passes in front of the other, as seen by the observer.

  • X-ray
    variable X-ray emission, usually from neutron star or black hole component of a binary, and often optically variable too.

  • Unique
    Unclassifiable & generally wierd variables!
Numerous subcategories have been defined on the basis of their characteristic variations, spectral features, or associations with other objects. Read How To Decipher Those Classification Codes for details.

How Much Equipment do I Need to Observe Variables?

 Not much for visual observations: a notebook and pencil, a watch, chart(s) for the variable(s) of interest; and it's helpful to have optical aid; especially if you live in suburbia. A few variable stars can be followed entirely with the naked eye. 7x50 binoculars can observe over 100 variables. Almost all of the 4,000 or so variable stars charted by the VSS RASNZ, BAA and AAVSO can be monitored with a 20cm telescope.

The best equipment is simple equipment that you are totally familiar with, so you don't waste time fumbling in the dark. Knowing your way around the sky is a big help, and a good general star atlas will help you find fainter variables.

A telescope should be equipped with either a good finder scope, or an eyepiece which gives 20-40x magnification. Either of these will speed up the locating of variables' starfields. Higher magnification eyepiece(s) will aid recognition in crowded starfields.

Ideally, a reflector telescope should have a slightly "oversize" secondary mirror to ensure a fully illuminated field of view. So-called "Planetary scopes" are designed with the smallest possible secondary mirror so that they achieve maximum resolution. This may come at the expense of a fully illuminated field of view. A quick check for this is to watch a star as it drifts through the field of view - if the star is brighter in the centre of the field, then the secondary mirror is too small to provide a fully illuminated field of view.

If the star's image changes shape during this same check, then your telescope and/or eyepiece is optically misaligned (at best) or optically defective (at worst). Some optical problems are fixable, but if you don't know what to do, get help from someone who does!

Do I Need Super Eyesight to Distinguish 0.001mag Changes?

 No human retina is that good, and no-one expects it to be that good. That is why visual observers don't watch stars expected to vary less than 0.4 mags. Experienced visual observers can see 0.1mag differences under ideal conditions, and 0.2mag more typically. It is normal for visual observers to have small systematic disagreements with photometer/CCD measurements - and with each other! - but pooling of observations produces a mean daily (weekly/hourly) magnitude virtually identical to an electronic device.

Interestingly, more than half of the world's regular variable star observers wear spectacles or contact lenses!

Do I Need a Dark Site Unobstructed in All Directions?

 No, but it's better if you do. Indirect light (eg: skyglow) can be overcome by increasing magnification; for this reason I strongly recommend suburban observers use binoculars at least. Direct light is bad; shield the light, kill the light, or move out of its illumination. A Full Moon can be stopped with a well-placed tree or an umbrella.

The International Dark-Sky Association (IDA) is one of several nonprofit organizations working to save our view of the Universe - and save money - by promoting efficient and effective nocturnal lighting. IDA's site offers tons of information about light pollution and what you can do to stop it.

Which is the Best Star Atlas to Use When Finding Variables?

 A matter of personal preference. I use the field edition (white stars on black paper) of Sky Atlas 2000 for three reasons: it's cheap, each page shows a big piece of sky, and virtually every variable star chart contains at least one star that also appears in Sky Atlas. For variables in the Magellanic Clouds I use Morel's Visual Atlases of the LMC & SMC which are simply excellent. Other good atlases include Uranometria 2000, and (for binocular users) Tirion's Bright Star Atlas or the Cambridge Star Atlas (also by Tirion).

None of these atlases depict comparison star sequences; so you will still need the detailed chart(s) for a variable in order to observe it.

The AAVSO Variable Star Atlas does include comparison star sequences. Unfortunately many of these are outdated - even in the 2nd edition - and the atlas itself is not easy to read in red light. A shame really, as this Atlas alone would suffice for all observations brighter than magnitude 10.0.

The Herald-Bobroff Astroatlas depicts magnitudes for its brighter stars; plus so much additional information that it can get hard to read under a red light!

Computer star atlases with good variable star charting facilities include: There are others; but I haven't seen them in action. My personal favourite is GUIDE.

What Variables can be Observed on Moonlit Nights?

 Generally, red variables should not be observed if the Moon is above the horizon and between first and last quarter phase. Red variables include Miras, semiregulars, RV Tauri stars and slow irregular variables. R Coronae Borealis and Z Andromedae variables are also red but should be observed regardless of moonlight in order to spot any sudden changes in brightness. Novae and dwarf novae, x-ray variables, and eclipsing binaries can be observed regardless of moonlight. Cepheids (all types), S Doradus stars and young irregular (Orion-type) variables are preferably not observed during full moonlight.

What Happens to my Observations?

 They are wasted if they go no further than your notebook. Submit them to one of the international variable star organizations. Your observations are preserved and made available to astronomical researchers worldwide.

Amateur visual observations have proved and disproved many a professional theory. Amateur observers usually have exclusive use of their telescope, and can spend as long as they want on an observing project; advantages denied to most professional astronomers.

Another important service is alerts - "this star is doing something unusual" - for the astronomical community. This allows timely observations of transient phenomena by scarce and expensive professional equipment; eg: satellite telescopes.

What is the Best Strategy for Observing Variables?

 It depends on your personal situation. Some observers look at a couple of dozen stars several times per month. Others concentrate on one class of variable. Some do all the variables in certain constellations only. A couple of observers with big telescopes observe only variables at minimum light because few other observers can see them.

Work out how frequently you can observe, how faint you can see, and which bits of the sky are accessible to you. Then select a list of stars to observe.

Certain classes of variables demand special strategies. Periodic variables in general need to be observed about ten times per period to define a meaningful lightcurve. Most eclipsing binaries require intensive observation during eclipses and little or no attention at other times. Dwarf novae and most eruptive variables require nightly observations. However, only YOU can decide how many stars you can observe

Where/How do I Start Observing Variables?

 Make the effort to get outside on clear nights and look up. Learn your night sky. Then practice finding the starfield containing the variable. Once you are confident of identifying the field, the comparison stars, and the variable (if visible); then start doing magnitude estimates. Don't forget to record your estimate, the date/time it was made, and the star's name.

Your first targets should be bright and/or easily located. A lot of my variables are near bright stars or distinctive star patterns; thus easy to find with my noncomputerized altazimuth telescope or binoculars.

Of course one of the best ways to get started is to get hold of a variable star mentor - and you know where to find me!

How do I search for novae/supernovae?

 Don't even think about doing this if you are a beginner!

If you want to find new supernovae, read the excellent advice from the International Supernova Network. This was written by active supernova hunters who know their stuff. You will be facing intense and well-equipped competition, including fully automated professional telescopes which can check hundreds of galaxies per night!

Note, however, that the brightest supernova this century - SN1987A - was found independently by three naked-eye observers. It takes only a few moments to check the Magellanic Clouds....

To discover a typical nova, you will need as an absolute minimum:
  • A camera on an equatorial mounting (so it can track the sky and take exposures lasting for many minutes). Most searchers use a 35mm camera with a 135 to 400mm lens; and expose long enough to record 11th magnitude stars.


  • Equipment and chemicals to quickly develop your own films. ASA 400 films offer a good compromise between graininess and film speed. Black and white films and slide films are used by many searchers.


  • Some way to compare your photo(s) with your previous photo(s) of the same bit of sky, so that you can spot any changes. The ProBliCon technique - where two slide projectors are used to project both images onto the same screen - is one way to compare images on slides. Some nova hunters have built themselves a blink comparator device to compare photos. Bear in mind that each photo might record up to 100,000 stars and other objects!


  • A notebook to record every detail of every photograph: date, time, exposure time, sky coordinates, the camera and film used, weather conditions, and more.


  • Detailed star atlases and catalogues, so that you can check out suspects, and measure their positions to within a few minutes of arc. You will discover hundreds (if not thousands) of known variable stars and asteroids on your photos; as well as nebulae, clusters, and the odd comet or two.

    Many nova hunters now use one of the computer star atlases (eg: GUIDE, The_Sky, or VoyagerII) to check for known objects. Internet resources include online versions of the Digital Sky Survey, the USNO A2.0 Catalogue, and ephemerides for all numbered asteroids and comets. However, the Perfect Star Atlas has yet to be compiled - objects do get omitted from catalogues, and typographical errors do occur. Computer star atlases also suffer the problems of round-off errors in catalogue positions, and artifacts of the compilation process.

    A wise nova hunter will also check a suspect by taking a second photo as soon as possible; and by visually observing the suspect. Modern cameras and films are not 100% perfect - many would-be discoverers have embarrassed themselves by reporting dust specks and film defects as novae. Asteroids usually reveal themselves by their movement between the times of the two photos.


  • A way to rapidly communicate your possible discovery. The Central Bureau for Astronomical Telegrams collects reports of new discoveries; and (if they are real) officially informs the astronomical community. Confirmation by an independent observer is a prerequisite for official recognition of a discovery.
When reporting a discovery, the following minimum information should be supplied:
  • Your name, your location, and details of how to contact you.


  • The position of the object, as accurately as you can measure it. Other observers need this to find and confirm your discovery. If you are measuring from a star atlas, mention the epoch (1950 or 2000) of the atlas.

    Don't use ridiculous levels of precision, especially if you're measuring from a wide-field photo or a pencilled "X" in your star atlas. Giving a position to the nearest minute of RA and the nearest arcminute of Dec is usually good enough.


  • The date(s) and time(s) the object was seen/photographed. Universal Time (UT) is preferred.


  • Brief description of the equipment used for the discovery (eg: camera, film and exposure data).


  • The approximate magnitude of the object. This is difficult to determine from film; the best way is to observe it visually.


  • Why you believe this is a new object. For example: object not seen on previous night's photo, or no known variables/asteroids at this position. Mention the atlases, catalogues, charts or software you used to check for known objects.
Most "discoveries" turn out to be errors - even outright frauds - so expect scepticism and very detailed questioning about your discovery. The Central Bureau for Astronomical Telegrams Web site contains further advice.

There are relatively few nova hunters in the southern hemisphere. The best regions to search are along the Milky Way (up to 20 degrees either side of the galactic plane), and the two Magellanic Clouds. Concentrate on the sky south of declination -10 degrees. The northern hemisphere contains many busy nova hunters, most of whom can see down to this declination.

The Clouds will require longer exposures because their novae rarely exceed magnitude 10.5. The best atlases of the Clouds (for amateurs) are produced by Morel Astrographics.

Nova hunters also discover previously unknown variable stars (non-nova), some of which turn out to be interesting in their own right. There have also been many new asteroids and quite a few new comets found during nova searches.
Page Address: http://www.assa.org.au/sig/variables/faq.asp