Re: Firing up
Unh. So if a star blows off its outer layers and the remnant is smaller than ten Jupiters, then it becomes a planet?
No. You're confusing smaller with less massive.
All known white dwarfs have a mass of between 0.5 and 1.44 solar masses, yet their radius is no more than that of the Earth. The Chandrasekhar Limit of 1.44 solar masses is accepted as the theoretical maximum mass for a standard electron-degeneracy white dwarf and is supported by observation, while the existence of helium white dwarfs of below 0.5 solar masses is hypothesised but not yet supported by observation (possibly because the hypothesis also suggests that it would take longer than the current age of the universe for a candidate star to reach this stage of its life).
Stars can shine, planets only cool. Simple.
No, it's really not that simple. White dwarfs do only cool (unless they acquire sufficiently more mass from somewhere, anyway). They are not producing heat by fusion; they are extremely hot stellar cores radiating inherent heat and will take tens of billions of years to reach background temperature. Rocky planets, on the other hand, are still capable of generating heat and radiating it away, although by nuclear fission rather than fusion (and don't get me started on gas giants).