Many thanks to Maura McLaughlin, Michael Kramer and Ingrid Stairs who read and provided very
useful comments on this revised review, as well as Paul Ray and Matthew Bailes for spotting errors in the
previous version. I am indebted to a number of colleagues who, as noted in many of the figure captions,
kindly gave permission to use their figures in this article. Frequent use was made of NASA’s magnificent
Astrophysics Data System , the arXiv e-Print service  and of course Google during the literature
searches. The Royal Society supports my research through their University Research Fellowship scheme.
Finally, I’d like to thank the Living Reviews editors for their support and patience during the completion of
this long-overdue update.