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Cepheid variables

You should know about the use of the parallax of a star to find its distance from the Earth. This is very useful for relatively near stars but for more distant stars and certainly for objects outside our galaxy the angles are so tiny that they are almost impossible to measure. Another method had to be found.

The solution came early in the twentieth century as a result of studies of a variable star (one whose brightness changes with time) in the constellation of Cepheus. The brightness of the star varied in a particular way. Many other stars were found to vary in a similar way and the group of stars was called Cepheid variables.

Over a period of a few days the size of the star changed and as it did so its brightness also changed (see Figure 1 notice that the star was not at its brightest when its size was greatest).

In 1912 Henrietta Leavitt, an astronomer at Harvard College observatory, discovered an important connection between the period (the time between moments of maximum brightness) and its actual brightness.

This is now known as the period-luminosity relationship.

So you first measure the period of the star and from the period-luminosity relation you can find out how bright the star really is.

Knowing how bright the star really is and then measuring how bright it appears to be you can then work out the distance of the star from the Earth. The discovery of Cepheid variables in the Andromeda nebula (M31) enabled its distance from Earth (over two million light years) to be found.


© Keith Gibbs 2020