In a unique collaboration, Nature Publishing Group and Institute of Physics Publishing have published the most extensive and comprehensive reference work in astronomy and astrophysics in both print and online formats. Unfortunately the print edition is currently out of print, but the regularly updated web remains active and can be found at www.ency-astro.com. This unique resource covers the entire field of astronomy and astrophysics and this online version includes the full text of over 2,750 articles, plus sophisticated search and retrieval functionality, links to the primary literature, and is frequently updated with new material. An active editorial team, headed by the Encyclopaedia's editor-in-chief, Maul Murdin, oversee the continual commissioning, reviewing and loading of new and revised content. The Encyclopaedia's authority is assured by editorial and advisory boards drawn from the world's foremost astronomers and astrophysicists. This first class resource will be an essential source of information for undergraduates, graduate students, researchers and seasoned professionals, as well as for committed amateurs, librarians and lay people wishing to consult the definitive astronomy and astrophysics reference work.