and
type II Cepheids On September 10, 1784,
Edward Pigott detected the variability of
Eta Aquilae, the first known representative of the class of classical Cepheid variables. The eponymous star for classical Cepheids,
Delta Cephei, was discovered to be variable by
John Goodricke a few months later. The number of similar variables grew to several dozen by the end of the 19th century, and they were referred to as a class as Cepheids. Most of the Cepheids were known from the distinctive light curve shapes with the rapid increase in brightness and a hump, but some with more symmetrical light curves were known as Geminids after the prototype
ζ Geminorum. A relationship between the period and luminosity for classical Cepheids was discovered in 1908 by
Henrietta Swan Leavitt in an investigation of thousands of variable stars in the
Magellanic Clouds. She published it in 1912 with further evidence. Cepheid variables were found to show
radial velocity variation with the same period as the luminosity variation, and initially this was interpreted as evidence that these stars were part of a
binary system. However, in 1914,
Harlow Shapley demonstrated that this idea should be abandoned. Two years later, Shapley and others had discovered that Cepheid variables changed their
spectral types over the course of a cycle. In 1913,
Ejnar Hertzsprung attempted to find distances to 13 Cepheids using their motion through the sky. (His results would later require revision.) In 1918, Harlow Shapley used Cepheids to place initial constraints on the size and shape of the
Milky Way and of the placement of the Sun within it. In 1924,
Edwin Hubble established the distance to classical Cepheid variables in the
Andromeda Galaxy, until then known as the "Andromeda
Nebula" and showed that those variables were not members of the Milky Way. Hubble's finding settled the question raised in the "
Great Debate" of whether the Milky Way represented the entire Universe or was merely one of many
galaxies in the Universe. In 1929, Hubble and
Milton L. Humason formulated what is now known as
Hubble's law by combining Cepheid distances to several galaxies with
Vesto Slipher's measurements of the speed at which those galaxies recede from us. They discovered that
the Universe is expanding, confirming the theories of
Georges Lemaître. In the mid 20th century, significant problems with the astronomical distance scale were resolved by dividing the Cepheids into different classes with very different properties. In the 1940s,
Walter Baade recognized two separate populations of Cepheids (classical and type II). Classical Cepheids are younger and more massive population I stars, whereas type II Cepheids are older, fainter Population II stars. RR Lyrae stars, then known as Cluster Variables, were recognized fairly early as being a separate class of variable, due in part to their short periods. The mechanics of
stellar pulsation as a heat-engine was proposed in 1917 by
Arthur Stanley Eddington (who wrote at length on the dynamics of Cepheids), but it was not until 1953 that
S. A. Zhevakin identified ionized helium as a
likely valve for the engine. == Classes ==