| 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] |
| This is bookwork. Experience indicates that the main problem is that people do not always answer the question that they were actually asked! You need to ensure that you have discussed both the general requirements and the relevant properties of Cepheids. | [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] |
| Recall that one of the problems with obtaining an accurate value for H0 is the scatter caused by the individual "peculiar velocities" of galaxies, which are typically of order several hundred km/s. Considering a distance of 20 Mpc or so, what is the Hubble velocity? How does this compare? What do you conclude? | [1] |
(2005 Q5(a).)