Observing Variable Stars

Note: The material on this page was originally created by Fraser Farrell, with updates by David Benn and Robert Jenkins.

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. 

The number of recognised variable stars is growing. In 1948, 10,820 were listed in the Volume 1 of the General Catalogue of Variable Stars (GCVS). By mid-1998, the number of proven and suspected variables in our Galaxy was about 60,000 and in 2020 the number had grown to 72,058 listed in Volume 5.1 of the GCVS. In addition, 218,000 variable stars are listed in other catalogues. 

The statistics are not good. Only 5% of the 300 billion stars in our Galaxy are directly visible, many star fields 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, 4801 known variables are listed in the GCVS from the Large Magellanic Cloud - a number expected to increase enormously in the aftermath of the MACHO Project.

How much equipment do I need?

Visual Observing

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 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' star fields. Higher magnification eyepiece(s) will aid recognition in crowded star fields.

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!

Imaging

Many observers are moving to imaging the variables to enable lower magnitude observations and greater analysis of the data. Even so, many of the principles from Visual observing need to be followed. You need to choose your targets carefully, you need to have the right equipment (telescope and camera), you need to know your equipment and you need lots of patience.

Digital Single Lens Reflex (DSLR) cameras, and cooled CCD or the newer cooled CMOS cameras are all being used. 

The advantage with images is they can be stored indefinitely and reanalysed to check observations or provide historic data for newly identified variables. 

As with astroimaging, calibration frames (darks, bias and flat field frames) are essential for all observations. 

The big difference between visual and imaging is in the post observation work. Magnitude data is recorded on the night by visual observers while images have to be calibrated and analysed to mine the data. Imaged observations are analysed after the imaging run using software like MaxIm DL^® or VPhot available from AAVSO.

Many people who have started as astroimagers have moved variable star work as they have the equipment and they prefer the science of variable star work against the art of imaging. Again, the big difference is that astroimaging manipulates the images to obtain the most attractive image while variable star images cannot be manipulated or valuable data will be lost. 

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 imaging measurements - and with each other! - but pooling of observations produces a mean daily (weekly/hourly) magnitude virtually identical to an electronic device.

For most amateur imaging equipment, magnitudes less than 0.01 mags require lots of experience and lots of patience. 

When you start out, do not expect miracles. The best way with either visual or imaging is to observe popular variable stars and compare your results with others before loading them onto databases.  

Do I need a dark site?

No, but it's better if you do. 

For visual observers, 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.

Imaging equipment is very tolerant to suburban light pollution. The magnitudes are calculated from the image using differential photometry (the difference between a known star magnitude and the changing variable). As such the entire image is equally affected by the pollution and reduces errors.  

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, semi regulars, RV Tauri stars and slow irregular variables. R Coronae Borealis and Z Andromeda 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

There are many star atlases available and most telescopes have GoTo functions to assist observers find stars.  The AAVSO website, Pick-a-Star section enables observers to Create finder charts to assist in identifying the correct star.  

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 like AAVSO. Your observations are preserved and made available to astronomical researchers worldwide.

To load your observations onto the AAVSO site, you will need to join the AAVSO and be allocated an Observer Code.  

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.

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 and from various file and Internet accessible sources. 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.

Certain classes of variables demand special strategies. Periodic variables in general need to be observed about ten times per period to define a meaningful light curve. Most eclipsing binaries require intensive observation during eclipses (including 4-hour continuous runs) 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

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.

Let's get started

A good place to start id the “Getting Started” section on the AAVSO website. It has tutorials on variable stars, manuals, descriptions on what variable stars are and how to find a mentor.  Then, make the effort to get outside on clear nights and look up. Learn your night sky. Practice finding the star field containing the variable. Once you are confident of identifying the field, the comparison stars, and the variable (if visible); then start doing magnitude estimates (visual or imaging). 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. Stars that have a large magnitude variation ensure you can see something happening in a short period. With short period eclipsing binaries (period of less than 0.5 day) and short period variables can show significant variation in a single observation run. Short period pulsating variables such as Cepheids change brightness noticeably on the time scale of days while Delta Scutis change on timescales measured in hours, but the amplitude is smaller in the latter. Novae can go from not being visible in some amateur telescopes to being visible to the naked eye or in binoculars within a day. Other cataclysmic variables can show small magnitude changes on timescales of minutes.

Of course, one of the best ways to get started is to get hold of a variable star mentor! ASSA has several people who can help; contact Robert Jenkins for more details.