Standard Candles, Cepheid Variables and the expanding universe
The correlation between a Cepheid variable’s brightness and the period of its peak and trough of brightness makes this kind of star a standard candle for distance approximation in the universe.Types of Cepheids include Classical Cepheids (Population I Cepheids, Type I Cepheids, or Delta Cephei variables), Type II Cepheids (Population II Cepheids), Anomalous Cepheids, and Dwarf Cepheids.
Classical Cepheids are between four and ten times the size of the Sun and pulsate during a period ranging from days to months.
Type II Cepheids exhibit a pulsation ranging from a day to two months. Usually smaller than the Sun and metal poor.
Edward Pigott in 1784 documented the pulsation of Eta Aquilae, a Classical Cepheid variables. But the standard example is Delta Cephei, documented by John Goodricke shortly thereafter.
Edwin Hubble postulated standard candle distances to the Andromeda Galaxy in 1924. In this time, astronomers still struggled with the difference between the Milky Way and the possibility of a larger universe.
In 1929, Hubble published more research theorizing an expanding universe, based on Cepheid variable measurements in many more galaxies.