Although it may not be obvious at first sight, many stars vary in brightness over time. Some of these variable stars change magnitude on predictable timescales, others are s regular. Recording the variations is a rewarding and straightforward form of observing which ultimately helps decode how certain stars work. In this article we'll look at different types of variable, how to observe them and how to interpret the results. We'll also give you some examples to get you started.
A star's brightness is quantified by its magnitude. Some stars remain at constant magnitude, some vary a little and some vary a lot. Indeed, some stars become bright enough to change the visual appearance of their host constellation, such as Betelgeuse (Alpha (a) Orionis) and Mira (Omicron (o) Ceti) (see 'Six variable stars to get you started' on page 66). Variability can occur on a predictable basis or can be highly irregular. The majority of variable stars appear to vary indefinitely, but some vary just once, with extreme examples being supernovae.
Variation in magnitude is either caused by external factors or by internal changes within the star; those in the first group are known as extrinsic variables, while those in the second are called intrinsic variables. An eclipsing binary such as Algol (Beta (B) Persei) is an example of an extrinsic variable; its observed brightness variation is due to a dimmer star passing in front of a brighter one with a very predictable period.
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