Blue loop

The onset of helium shell burning causes a thermal pulse and in some cases this will cause the star to temporarily increase its temperature and execute a blue loop.

[5] Red supergiants are massive stars that have left the main sequence and greatly expanded and cooled.

Stars from the asymptotic giant branch that cross the instability strip during a blue loop are thought to become W Virginis variables.

More massive stars, crossing the instability strip during a blue loop from the red-giant branch, are thought to make up the δ Cephei variables.

Both types of star have luminous and unstable photospheres at this stage of their lives and often have the spectra of supergiants, although most are not massive enough to ever fuse carbon or reach a supernova.