Classical Cepheid (δ Cepheid) variables¶
Classical Cepheid variables, δ Cephei variables or simply Cepheids are intermediate- and high-mass stars (more than about four times as massive as the Sun) that become unstable while fusing helium into carbon in their cores. The pulsations are usually the radial fundamental or its first overtone, driven by helium going back and forth between its first and second ionization states.
Cepheids follow precise period-luminosity relations that are an important way of measuring distances. Edwin Hubble famously used observations of Cepheids in Andromeda and other galaxies to show that these “nebulae” had to be far outside the Milky Way, and certainly not part of it.