The definition of a variable star is simply a stellar body whose apparent magnitude (ie its brightness seen from Earth) changes – usually in a regular cycle.
Broadly speaking, these can be divided into two categories: stars with intrinsic variation whose brightness actually varies because of its own physical makeup, and extrinsic variable stars whose variation is caused by other objects affecting the amount of light reaching Earth.
These two types can be further divided into dozens of subcategories. Intrinsic examples include Cepheids and Cepheid-like stars, a type of pulsating variable star, often with a regular cycle.
Eruptive variables experience changes in luminosity due to mass ejections or stellar eruptions on their surface so violent that the surge in energy output can be seen as a peak in apparent magnitude. Cataclysmic variables, meanwhile, can be considered the extreme end of eruptive variables, where the properties of the star are irrevocably changed as a result of a cataclysmic event, such as a nova or supernova.
Extrinsic variables, on the other hand, can be categorized into two main types. Rotating variables include stars that exhibit changes in luminosity because brighter and darker areas of their surface move in and out of view as they spin. This can be the result of a cluster of stellar spots or changes in the magnetic field over specific parts of the body’s surface. Eclipsing variables, meanwhile, often have a companion binary star that blocks some of its neighbour’s light when viewed from a certain angle.
A slight dip in their brightness might occur if a planet orbiting close to the star passes between it and Earth, such as exoplanet candidate UCF-1.01. This was discovered by NASA’s Spitzer Space Telescope in July 2012 orbiting the red dwarf GJ 436.
Distant stars and planetary systems are much too far away for us to take direct measurements, so discovering and observing Cepheid variable stars is vital because astronomers think they can use the periodicity of changes in Cepheid variable brightness to calculate their luminosity. If we know their luminosity then we can also measure their apparent magnitude. Once we have established both a star’s apparent magnitude and its luminosity, we can use those measurements to calculate its distance.
The way the information from variable stars is initially used by astronomers is a bit like a detective who is observing a crime scene to build a better idea of what happened. Apparent magnitude, luminosity and distance are all interrelated, so by closely observing the star we can obtain measurements for two of these values, then use them to calculate the third. This not only enables us to gain a better understanding of different types of star, but also helps us to create a much more accurate picture of the cosmos in general.