What Are Variable Stars?

Variable stars are, quite simply, stars that change brightness. More than 30,000 are known and catalogued, and many thousands more are suspected variables. Their light variations are a vital clue to their nature, but professional astronomers do not have the time or the telescopes to monitor the brightness of thousands of variables night after night; their efforts are directed toward understanding specific aspects of the stars using such sophisticated instruments as spectrographs, photometers, radio telescopes, and satellites. Such instruments enable us to probe many different regions of the spectrum, such as X-ray, ultraviolet, infrared, and radio wavelengths. Yet it is crucial to measure the stars in ordinary visual light (the optical region of the spectrum) as well. The data obtained with special instruments and in shorter or longer wavelengths are compared with optical measurements, which serve as a benchmark. By observing variable stars, a serious observer (such as an amateur astronomer or student) can make a significant contribution to astronomy. Also, to understand and create theories about why and how stars vary, astronomers need to know the long-term history of the stars, hence it is essential that we have long-term observations. To date, the vast majority of long-term data has been provided by amateur observers.

Observations of variable stars are plotted on a graph called a light curve as the apparent brightness (magnitude) versus time, usually in Julian Date (JD). The light curve is the single most important graph in variable star astronomy. The light curve allows astronomers to unlock some of the secrets of stars and decode the messages hidden within the starlight. Information about the periodic behavior, the orbital period of eclipsing binaries, or the regularity (or lack) of stellar eruptions can be directly determined from the light curve. More detailed analysis of the light curve allows astronomers to calculate such information as the masses or sizes of stars. Several years' worth of observational data can reveal the changing period of a star, which is a signal of a change in the structure of the star. In the same way that a histogram is a useful tool to inspect the precision of a set of measurements, we can visualize the nature of a star's variation more easily by plotting a light curve of apparent magnitude versus time.

Light curves show that many variable stars are periodic. Periodic phenomena repeat in a regular way that is predictable. However, the line between periodic phenomena and non-periodic phenomena is not always a sharp one. Sometimes a process may seem to be periodic from a large-scale perspective, while investigation on a fine scale reveals nonperiodic variations. Consider the change in apparent altitude of the Sun. If you measure the highest apparent altitude of the Sun each day for a week, you will not notice much change. The Sun seems to reach the same maximum altitude each day. Over a period of a month, however, changes will become obvious. But if we take measurements each day from December 31st until the summer solstice around June 21st, the apparent daily altitude of the Sun becomes higher and higher. This is still non-periodic behavior. If we take our measurements for an entire year, from December 31st to December 31st, we will see a definite pattern. When we examine the change in the apparent altitude of the Sun over several years, we see that in addition to monthly variations, it follows a regular pattern with a period of one year.

Many variable stars do exhibit simple periodic behavior. Some semiregular variables have time intervals of periodic behavior in between non-periodic intervals. Some have two separate periodic cycles superimposed on each other, affecting the overall result. The only way to determine the periodicity is by plotting and analyzing light curves. The emerging patterns will then tell their story.

There are four main classes of variable stars: pulsating and eruptive variables, whose variability is intrinsic - due to physical changes in the star or stellar system; and eclipsing binary and rotating stars, whose variability is extrinsic-due to the eclipse of one star by another or the effect of stellar rotation.