Quote:I am curious...the central star of planetary nebula; what are they? Are they still doing nuclear fusion, or are they a glowing remnant?
Quote:The exposed stellar core illuminating the planetary is already by that point a white dwarf, being typically about the size of Earth. Fusion has ceased (or at least soon will?), and the object will cool over many billions of years to become an invisible 'black' dwarf.
The reason for the dominance of O-III emission in most planetaries is the combination of oxygen produced as a byproduct of fusion, the very high UV flux from the hot stellar core, and the just right density of the gas.
The planetary phase is extremely brief, cosmically. After only a few tens of thousands of years the nebula will puff up and fade to invisibility.
Is it (ever) the case that the core temperature of stars which are no longer fusing, are hotter than the core temperature of stars which are still fusing?
Quote:One off-topic, but related question....is a Wolf Rayet star a star in which the nuclear-"furnace" is directly exposed to space; there is no "plasma cloud" surrounding the nuclear furnace? Is there any such object of which we know?