Recent developments concerning the Crab Nebula

This review is concerned with supernova (SN) ejecta now seen as thermal gas in the Crab Nebula. Some background information about the Crab is provided. The distance of the Crab Nebula is between 1500 and 2200 pc, and most of the nebular mass is in gaseous filaments with emission-line spectra. The spectroscopic determinations of the nebular mass and chemical composition are reviewed, taking into account emission-line observations, composition analyses, dust, differences among filaments, and the need for more observational abundance studies and better ionization models. The pre-SN star and the SN event are discussed, and some morphological questions are explored. Attention is given to the star's mass (actually its core mass), the supernova event, certain aspects which are not yet very well understood, the filamentary structure, and questions regarding a halo. 168 references