| Briefly explain why classical Cepheid variables are the most appropriate calibrators for many extragalactic distance indicators. You should start by explaining the requirements for such calibrators, and then explain how Cepheids fulfil these requirements. | [4] |
| Given the good qualities of Cepheids, why are they not used directly to measure the Hubble constant, but only as calibrators for other methods? | [1] |
(2005 Q5(a).)