Distances to Galaxies | Cepheid Variables | How to Look for Cepheid Variables | The Period-Luminosity Relation | Find the Cepheids in M100

4. The Period-Luminosity Relation

In order to find the distance to M100, you need to find the absolute magnitudes for each of the Cepheids. Since the luminosity of a star is related to its absolute visual magnitude (M), we can express the Period (P) -Luminosity (L) relationship as a P-M relationship. The P-M relationship for M100 is shown graphically below:

For each Cepheid you discover, use the above graph to determine the absolute magnitude of the Cepheid from its period. Remember! To get the distance to an object you need to know both its absolute magnitude AND its apparent magnitude. The Cepheid hunt will allow you to determine the apparent magnitude and the period. Using the above graph you can convert the period into an absolute magnitude. You will then have both quantities needed to find the distance, using the dimming factor conversion table between the difference of the apparent and absolute magnitudes and the distance, as we  used in the Clusters lab.