Period-luminosity relation

[2][3][4] Discovered in 1908 by Henrietta Swan Leavitt, the relation established Cepheids as foundational indicators of cosmic benchmarks for scaling galactic and extragalactic distances.

In 1908 she published her results in the Annals of the Astronomical Observatory of Harvard College, noting that the brighter variables had the longer period.

[13] Building on this work, Leavitt looked carefully at the relation between the periods and the brightness of a sample of 25 of the Cepheids variables in the Small Magellanic Cloud, published in 1912.

In the 1912 paper, Leavitt graphed the stellar magnitude versus the logarithm of the period and determined that, in her own words, A straight line can be readily drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the Cepheid variables and their periods.

[18] Also, in 2008, ESO astronomers estimated with a precision within 1% the distance to the Cepheid RS Puppis, using light echos from a nebula in which it is embedded.

[23] Most of the Cepheids were identified by the distinctive light curve shape with a rapid increase in brightness and a sharp turnover.

[25] These Cepheids are yellow bright giants and supergiants of spectral class F6 – K2 and their radii change by of the order of 10% during a pulsation cycle.

Hubble often said that Leavitt deserved the Nobel Prize for her work,[28] and indeed she was nominated by a member of the Swedish Academy of Sciences in 1924, although as she had died of cancer three years earlier she was not eligible.