Observing Variable Stars

Note: The material on this page was originally created by Fraser Farrell, with minor updates by David Benn (Nov 2012).

Amateur variable star observers are more important than ever because they 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.

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.

As of August 2012, the AAVSO International Database contains observations for more than 17,500 stars. 

How much equipment do I need?

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 17,500+ variable stars charted by the 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 magnitude 0.001 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 0.1 mag differences under ideal conditions, and 0.2 mag 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?

No, but it's better if you do. Indirect light (e.g. 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.

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. 

Star atlases

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. Use the AAVSO's VSP to create such charts.

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!

There are also plenty of computer star atlases with good variable star charting facilities.

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.

Unless involved with a special observing project, you will ordinarily submit your observations via AAVSO's WebObs.

Software for analysing observations

VStar is a multi-platform, easy-to-use variable star data visualization and analysis tool that was developed as part of the Citizen Sky project. Data for a star can be read from the AAVSO International database or from a text file of your own creation. Light curves and phase plots can be created and period analysis can be carried out. 

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

Let's get started

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!