Galaxies, Page 80: [Previous Page] [Next Page] [Class Home Page] Textbook pages 420-422

Variable Stars used to obtain Distances

It was determined early in the 20th century by Henrietta Leavitt that some stars varied in luminosity, and this change in luminosity had a specific repeating period.







The two most important variable stars showing this very regular, periodic variability are known as RR Lyrae stars and Cepheids . These stars truly do pulsate, expanding and contracting with a rythmic beat. Here is an animation.


But the final VERY important piece of information: The Period of this Luminosity change was related to their Luminosity. Very bright variable stars oscillated in brightness very slowly, while the dimmer variable stars oscillated in brightness very quickly, as shown below.


Period-Luminosity Relation for Variable Stars


Shapley was aware of Henrietta Leavitt's work on characterizing Variable Stars, and was able to obtain distances for a few. This allowed him to calibrate the system, making all variable stars useful for distance determinations.