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Annotated HR diagram showing the path of a five solar mass star as it leaves the Main Sequence. The Main Sequence is indicated by a diagonal orange line running from top left to bottom right. From the centre of the main sequence an elliptical white zone with a dotted outline extends towards the upper right of the diagram. This outer zone is labelled as the Instability strip. Two smaller zones within the instability strip are highlighted in orange – a smaller zone towards the bottom labelled 'RR Lyrae' and a larger elliptical zone at the top labelled 'Cepheids'. The path of a five solar mass star is shown as a red line extending horizontally to the right from a point about three-quarters of the way up the Main Sequence towards the centre of the Instability strip. This red line passes back and forth through the lower part of the orange Cepheid region before continuing towards the upper right, red giant area of the diagram.

 1.4 The instability strip