Their distinguishing characteristic results from the conversion of helium plus ions into alpha parti
Their distinguishing characteristic results from the conversion of helium plus ions into alpha particles in their atmospheres.  This causes their atmospheres to expand until the reverse reaction takes over and restores their size to their initial magnitudes.  They were discovered in 1784 by John Goodricke, and Harlow Shapley employed them to show that the Magellanic Clouds are not part of the Milky Way by using their characteristic period-luminosity relationship, which was first demonstrated by Henrietta Leavitt.  FTP, what are these stars useful in measuring intergalactic distances and whose prototypical example is Delta Cephei?
Their distinguishing characteristic results from the conversion of helium plus ions into alpha particles in their atmospheres.  
cepheid variables