Like Joe I wondered if the central stars of planetary nebulae were white dwarves so I went to google to get the truth. Turns out to be more complex than I thought. The Crab Nebula is a supernova remnant and so isn't a planetary nebula. The pulsar requires a supernova and the remnant is not a planetary. I found a few sources who mention that the "classical" view is that the central stars of pn's are "precursors" to white dwarves but none mentioned a spectral class for the type. A few pn's are proving to have Wolfe Rayet stars in the center. There are the exposed cores of very heavy stars that are destined to explode as supernovae in the future. The central star of Eta Carina is a star if this type. The argument then becomes that these pn's aren't "genuine" pn's because of the size class of their central stars and their ultimate fate. Sound familiar? DT