Silent Sky chronicles Henrietta Leavitt’s most famous contribution to astronomy: the Cepheid variable period-luminosity relationship, which discovered in 1912. But what does this mean? It is explained a bit in the play, but if you’re not an astronomy person, then it may take more than a quick snip-it of dialogue to fully understand it. So here is a breakdown of this relationship.

Overall, Henrietta was looking at the overall luminosity (or brightness) of the Cepheids in the Small and Large Magellanic Clouds and found a pattern between the luminosity and the pulsations. In the play, this pattern is related to music, but the actual relationship is not quite as rhythmically interesting. However, the rest of the information in the play is accurate. Henrietta discovered that the more luminous the Cepheid is, the slower it pulses. That is the most simple explanation of her breakthrough, but of course, there are many more factors involved in her overall discovery.

Because of the relative ease in which one can find this relationship, Cepheids can be used to measure the distance to other galaxies. For example, Edwin Hubble used Henrietta’s discovery to measure the distance to the Andromeda Galaxy.

While the this is the most basic information about the Cepheids, it is a simple introduction to the subject matter and is the only relevant information you need to fully understand Silent Sky. If you would like any more information on period-luminosity relationship or Cepheids in general, here are a couple of great resources:

http://www.atnf.csiro.au/outreach/education/senior/astrophysics/variable_cepheids.html

http://astro.wku.edu/labs/m100/PLrelation.html