Cepheid Variable - History

History

On September 10, 1784, Edward Pigott detected the variability of Eta Aquilae, the first known representative of the class of Classical Cepheid variables. However, the namesake for classical Cepheids is the star Delta Cephei, discovered to be variable by John Goodricke a few months later.

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.

In 1913, Ejnar Hertzsprung did some work on Cepheids, which needed some revision.

In 1915, Harlow Shapley used Cepheids to place initial constraints on the size and shape of the Milky Way, and of the placement of our Sun within it.

In 1924, Edwin Hubble established the distance to Classical Cepheid variables in the Andromeda Galaxy, and showed that the variables were not members of the Milky Way. That settled the Island Universe debate which was concerned with whether the Milky Way and the Universe were synonymous, or was the Milky Way merely one in a plethora of galaxies that constitute 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 (see the expansion of the Universe). However, the expansion of the Universe was posited several years before by 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. Classical Cepheids and Type II Cepheids follow different period-luminosity relationships. The luminosity of Type II Cepheids is, on average, less than classical Cepheids by about 1.5 magnitudes (but still brighter than RR Lyrae stars). Initial studies of Cepheid variable distances were complicated by the inadvertent admixture of classical Cepheids and Type II Cepheids. Walter Baade's seminal discovery led to a fourfold increase in the distance to M31, and the extragalactic distance scale. RR Lyrae stars were recognized fairly early (by the 1930s) as being a separate class of variable, due in part to their short periods.