Stars that change in brightness are referred to as variable stars. The discovery and study of variable stars are activities in which amateur astronomers can make contributions that help professional astronomers. These stars are usually presented by plotting the brightness versus time in graphic form known as a light curve.
The first step in organizing what we know about variable stars is to classify them into two broad categories: 1) the regular or predictable type and 2) the irregular or unpredictable type. Those stars that are considered regular variables and have periods of hours to about a hundred days are called short period variables. Those stars that are over a hundred days are called long period variables. Flare stars, nova, and supernova are within the group classified as irregular and are what might be called eruptive type.
The change in brightness of a variable star is usually expressed using the magnitude scale. The difference in magnitude (∆m) between maximum and minimum brightness is referred to as amplitude. Since the change in magnitude is not the actual difference in light intensity, we must convert from the magnitude scale to brightness ratio. A difference of one magnitude corresponds to a brightness ratio of about 2.5X, thus indicating a light intensity change of 2.5X.
Once a star is recognized as a possible variable star, it is carefully studied over an extended period of time, until its period and amplitude can be confirmed, then it is named. Variable stars are usually divided into three major categories: pulsating variables, eruptive variables and eclipsing binaries. Regular and semiregular variables generally are aging, pulsating stars that have evolved from the main sequence of the HR Diagram into a region called the instability strip. This strip marks a region of the HR Diagram roughly bounded by spectral classes A through G and absolute magnitudes +3 to -5.
Brightness of Stars and Stellar Magnitudes