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In a universe where a lot of things are unclear, it’s good if we have a few measuring instruments that behave in predictable ways. The Cepheids – variable stars – are such a measuring instrument.
In the constellation Cepheus, just above Cassiopeia, about 887 light years away is the star Delta Cephei. It has been known since 1784 that it is a variable star: over a period of a few days, the star’s luminosity increases and then decreases again. This pattern repeats every 5.37 days.
Thousands of variable stars
In the early 20th century, it became clear that there were many more stars like Delta Cephei. Research on thousands of stars has shown that there is a clear correlation between the period of the pulsation and the brightness of stars.
Why are Cepheids such a reliable measuring instrument? Two things can be measured very precisely in such a star: the period of the pulsation, and the luminosity. But this luminosity depends on the distance to the star, and therefore relative. With this knowledge, we can therefore determine exactly how far each variable star is from Earth.
expanding universe
Therefore, the Cepheids are like landmarks in the starry sky, with which we can determine all kinds of distances. This brought many benefits to science – for example, in 1929 astronomers were able to determine that the universe must expand.
Do you want to know exactly why Cepheids pulsate? Then listen to Hens Zimmerman’s column.
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