Classical Cepheids ( also known as Population I Cepheids , Type I Cepheids , or Delta Cepheid variables ) undergo pulsations with very regular periods on the order of days to months .
Classical Cepheids ( also known as Population I Cepheids , Type I Cepheids , or Delta Cephei variables ) pulsate with very regular periods of the order of days to months .