BASIC OBSERVING SERIES

by Greg Burnett (gburnett)

Variable Stars

[This article was first published in Star Stuff, the newsletter of the Ford Amateur Astronomy Club, in September, 1993.]

The ancients believed that the sky was perfectly constant, reflecting a notion god-like perpetuity. When they discovered planets, they were naturally uncomfortable, and invented wonderful rationalizations for these "wanderers". The discovery of variable stars put to rest forever the idea of eternally unchanging heavens.

Variable stars fall into four major physical classifications. "Pulsating variables" comprise about a dozen sub-types, including the Cepheid variables that have been so important in measuring the size of the universe. These are stars that exhibit varying brightness due to periodic changes in their internal structure. "Eruptive variables" include a dozen or so sub-types that are considered irregular variables, and also novae and supernovae. Novae are now understood to be members of binary systems that become unstable due to transfer of matter from one star to the other. Supernovae are the explosive results of instabilities in super-giant stars. "Eclipsing binaries" are simply double stars that happen to be oriented so that the stars eclipse on another as they orbit. We observe these eclipses as a variety of possible light curves that can tell us alot about the sizes and other properties of the two stars. "Peculiar variables" include whatever we cannot otherwise classify; there are still some mysteries out there.

The observation of variable stars is one of the last areas where the amateur can contribute observations with real scientific value. There are thousands of variables for which definite light curves and classifications have not yet been established simply because professional observatories cannot devote the required time to monitor them. Likewise, there are numerous irregular and suspected variables that will reveal themselves only if constantly watched. Large organizations of dedicated amateurs provide the only opportunity for sufficient observation of these stars. Similarly, amateurs are often the first discoverers of supernovae, although automated surveys are proving more and more productive.

Almost any equipment can be used to observe variables stars, even binoculars. There are many variables within reach of small telescopes. High quality optics are not a requirement, since resolution and image quality are not important factors. Larger apertures will gain access to fainter stars, including those most lacking regular observation.

Much more important than equipment is observing technique. Overcoming physiological obstacles to "point photometry" with the human eye requires a rigorous adherence to procedure and a great deal of practice to achieve accuracy. Making a variable star observation involves comparing the brightness of the variable to one or more comparison stars. In doing so, the observer must be sure to use the same area of their retina for both stars, since retinal sensitivity varies widely over the retina; averted vision should not be used unless absolutely necessary to see the star. When observing colored stars, the observer must be aware of the eye's non-instantaneous adaptation to red light. Stars with a strong component of red light in their spectrum will seem to become brighter as they are looked at longer. In addition, the "Purkinje effect" will cause red stars to appear brighter than they really are under certain comparison conditions.

There are two fundamental methods for performing magnitude comparisons. To insure accuracy, the observer should adopt a particular method and practice it on known stars until they are adept. One method is called the "step method." This method involves learning to discern magnitude differences of 0.1, 0.2,... all the way to one full magnitude. This must be learned independent of the absolute star brightness; it's the differences that are important. When observing with the step method, the variable is compared with a single comparison star, and the perceived number of 0.1 magnitude steps between the variable and the comparison star are noted. Several separate observations are made using different comparison stars. The observations are later reduced by looking up the catalog brightness of the comparison stars and determining the variable's brightness by adding or subtracting the appropriate multiples of 0.1 magnitude. Clearly, this method requires practice to achieve accurate results.

The second method is called the "fractional method." This method does not rely on learning fixed magnitude steps. It uses two comparison stars simultaneously, instead of just one as in the step method. When observing, the observer determines the ratio of differences between the variable and the two comparison stars. For example, the variable may lie two thirds of the way in brightness from the fainter comparison star to the brighter. The ratio 2:3 would be recorded (along with the identity of the comparison stars, of course). The ratio could vary from 1:1 to 3:5, 2:5, 1:5, etc. Using a denominator greater than 5 becomes difficult. The observations are later reduced by applying the ratio to the catalog brightness of the two comparison stars. This method may at first appear easier than the step method, but appearances are often deceptive.

In addition to a well-trained eye, careful record keeping is required for useful observations. Some variables change brightness slowly over periods of weeks or months. Useful light curves can be built up only from carefully recorded observations that include the observing conditions and the comparison method used. There are several worldwide organizations dedicated to collecting and utilizing amateur observations of variables stars. The most noteworthy are the American Association of Variable Star Observer (A.A.V.S.O.) and the Variable Star Section of the British Astronomical Association. Both of these organizations publish stars charts specifically designed for variable star observing, with comparison stars clearly marked.

Amateurs considering embarking on a variable star observing program would do well to consult one of the many books on the subject. Observational Astronomy for Amateurs by J.B. Sidgwick (Enslow Publishing) contains a chapter on variables that describes the step and fractional observation methods in detail. The Webb Society Deep-Sky Observer's Handbook, vol. 8: Variable Stars by Kenneth Glyn Jones (ed.) is perhaps the standard amateur reference on variables. Serious observers should obtain the AAVSO Variable Star Atlas (2nd ed.) by Charles Scovil. Catalogs of variable stars are included in most listings of popular amateur observing targets, for example Norton's Star Atlas and Sky Catalog 2000.0. All of these books are available from either Willman-Bell or Sky Publishing.