The brightness of certain stars varies over periods of minutes to years. Observing these variations can be quite challenging and fun. There are several kinds of variable stars. Mira or Long-period Variables are the most common type, and the type most easily observed.
Internal changes cause the star to pulsate in size and brightness over a period of weeks.
Its brightness varies by six orders of magnitude so that, to the naked eye, it seems to appear in the sky newly about once a year. Cepheid Variables and RR Lyrae Variables expand and contract in a similar way, but over a much shorter time span, from less than a day, up to a few days. Eclipsing Binaries are gravitationally bound pairs of stars whose brightness varies because one periodically eclipses the other. Definitely the best introduction to the subject of variable stars, written by a veteran observer, this book covers the different types of variable stars and other variable objects, and gives directions for observing them yourself. A brief guide to the history, naming and observation of variable stars, with the telescope and naked eye.