A star whose brightness changes is called a variable star.
Astronomers use a type of variable star called a Cepheid to measure distances in the Universe. There is a relationship between the length of time that it takes a Cepheid's brightness to change – its period – and its overall luminosity; Cepheids with longer periods are more luminous.
Astronomers compare the luminosity (calculated from the period) with how bright it appears in the sky to work out how far away the star is.
Image: V838 Monocerotis, a red variable star near our galaxy's edge (credit: Hubble Heritage Team, ESA, NASA)
Sir Patrick Moore explains how to estimate the magnitude of a variable star.
This clip was filmed at a time when the Hubble Space Telescope had been discovered to have a flawed mirror, hence the reference to "failure". The Hubble Space Telescope went on to become one of the most successful NASA missions. This clip looks at some of the American astronomer Edwin Hubble's most important contributions to astronomy and the equipment he used at the Mount Wilson Observatory. The age of the Universe is now thought to be 13.7 billion years.
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth. Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11 year solar cycle.
It is convenient to classify variable stars as belonging to one of two types:
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