Cepheids as Distance Markers

The important feature of a Cepheid Variable that allows it to be used for distance measurements is that its period is related directly to its luminosity . This relation allows us to work out how much brighter than the Sun the star is. From there we can calculate how much further away the star must be than the Sun to make it the brightness we see from Earth.

M51 - A closeby galaxy whose distance could be found using Cepheids

The only complication is that there are two types of Cepheids (Population 1 and Population 2). These have different relationships between their luminosity and period. Before the 1950s the measurements gave results for distances that we now know to be wrong. Therefore it is necessary to find out which type of star is being observed before this method can be used.

* Click here for more information on Cepheids

* Click here for an example of this method

* Click here for to find out about the magnitude scale.