The point is not that the carbon is in a plasma state: these stars are described as "white", thus white-hot, hotter than our sun which is yellowish. Of course carbon is in a plasma state of individual atoms at this temperature.
What surprises astronomers, if I understood the article correctly, is the following: when all the core hydrogen has been burned to helium, there are two normally possible outcomes for a star:
- heavy stars undergo helium-burning to carbon and oxygen, then go on to burn these elements to heavier ones, then collapse and explode back to supernova, leaving only a very small core (neutron star or black hole)
- lighter stars burn the core to carbon and oxygen and stop. Part of the outlying atmosphere is expelled but part of it is not, and what is seen is a
sufficiently thick layer of either hydrogen or helium totally hiding (one does not need much! a small percentage of the total mass will be enough to be totally opaque!) the core of carbon and oxygen, very hot (plasma state) since the core temperature is still higher than the surface: the enormous amount of heat that had been created while the core was still undergoing nuclear reactions has not yet had time to be conducted/convected to the outside, and will keep the outside hot for billions of years; then the star will finally cool and not be a "white dwarf" anymore!
But here one sees the carbon-rich core, for some reason that one does not yet understand, and that is different from these two processes: an event, not as dramatic as a supernova, but more violent than the usual expulsion of just the outermost layers that is expected in the formation of most white dwarfs, has occured: the
entire hydrogen or helium outer layer has been expelled, leaving just the central (very hot, in plasma form) carbon/oxygen core that is
expected to exist within all white stars. So the surprise is not the existence of the core, but the absence of the outer layers, which allows this core to be seen. This goes agaisnt "traditional wisdom" that some part of these layers normally remain, only part of them are expelled. Even a small amont is sufficient to be opaque. Here the entire outer part has been expelled, all the way to the carbon/oxygen rich core, and what is not understood is this "intermedate" situation, more violent than normal white-dwarf behaviour but much less than the supernova explosion.
It is surprising, but we do not understand all the fine details of star evolution. It is a challenge to find the appropriate model to explain just how it happened.
But the
existence of the (very hot, plasma) core of carbon/oxygen is not a paradox in itself. Just the fact that it is seen. In a sense, it even
confirms what was usually believed: such a core does exist in the inside of white dwarfs, as theories had predicted!
Among the many references of the
White Dwarf article in Wikipedia I found this one rather illuminating.
http://www.shef.ac.uk/physics/people/vd ... wmass.html
The outer part is lost in the "planetary nebula" stage. This reference does say this stage "is not well understood". The amount of hydrogen and/or helium that is
not expelled at this stage is not such an essential tenet of the theories. Though the newfound stars are surprising in the fact that hydrogen/helium expulson is total rather than partial, it does not seem to be a paradigm-shattering discovery. A suprise, yes, showing need to
refine the models, by all means, but not a dramatic destruction of all that was previously believed to be true.