Stars are like people. Just as there are, in a sense, billions of types of people, there are tens of thousands of types of variable stars. However hard we try to classify variables, we always run into exceptions, and when we create a new category for the exceptions to the old category, that usually turns out to have exceptions too. Although it is an exaggeration to say that every variable star in the sky is in a class by itself, it is useful to think of variables as individuals capable of surprises.

There is a different aspect, however, of the philosophy of classification. Just as we can find strength and beauty by looking at the diversity of language, behavior, and culture in people, a look at the kinds of variation in stars will help to show just how extraordinary this field of study really is.

Some classifications are obvious. You would never want to confuse a stately, mature, slowly varying red giant like Mira with a little dwarf star that erupts every two months. An Orion variable, changing brightness for no apparent rhyme or reason, would not be confused with a slightly unpredictable semiregular. The eclipsing binaries, revolving together in clockwork fashion, are not the same as the intricate ‘breathing’ of the Cepheids. Astronomers recognize these broad divisions that help us both to understand the different patterns of variation and to plan our observing programs for them.