I know of three mechanisms which can drive a stellar wind: (1) radiation, (2) Alfven waves, and (3) a magnetic rotor. Supermassive stars will shine bright enough to push the wind by radiation pressure. That works for solar type stars only when they enter the red giant stage of evolution, and get bright enough. An Alfven wave is a wave in the magnetic field, which will certainly drive charged particles, and is most likely the source for the acceleration of the solar wind. The magnetic rotor effect is based on the fact that a rotating magnetic field will transfer angular momentum to a stellar wind.

So it's not just "a" defining characteristic, but at least a few defining characteristics. Since spectral class, mass & temperature are related, it's no surprise that stellar winds will be related to spectral class too. So O, A & B stars will have faster and more massive stellar winds, in general. Wolf-rayet stars have huge stellar winds. Our own sun will probably lose about 20% of its mass to stellar winds before it settles into a long retirement as a white dwarf. A more massive star, say 5 solar masses, will probably lose over 80% of its mass to stellar winds, in its red & asymptotic giant phases. More massive, supermassive stars won't likley lose that much, but will probably lose about half their mass to stellar winds.

There is in fact a lot that is not known about stellar winds. There may be, and probably are, other mechanisms for driving winds (plain old thermal instabilities in pulsating stars?), it's a really complicated field. See Introduction to Stellar Winds, H.J.G.L.M. Lamers & J.P. Cassinelli (no typo there, Lamers has a lot of names), Cambridge university Press, 1999. It's the only full-length treatment of the topic that I know of, though there will be a chapter on winds in any book on stellar evolution or stellar atmospheres.

I don't see how they could be, but the standard explanation is perfectly acceptable, and explains why only Io connects to Jupiter electrically, and that's tidal flexing. Io is caught in a tidal squeeze between Jupiter and the other Galilean satellites. The flexing generates internal heat, which is expressed at the surface by volcanism. Io is the smallest of the Galiean satellites, and its the nearest one to Jupiter, which makes the effect greatest for Io. Europa is also tidally stressed, but it's bigger and farther away. So, instead of overt volcanism, the internal heating generates the stress fractures seen all over the surface (which form a pattern that focusses on the Jupiter-Europa axis). Ganymede & Callisto are too far out, and not subject to so much stress, since they don't have more satellites outside their orbits. I don't see a problem with this explanation.

But once the volcanos provide the sodium, it's not that hard to ionize, and Jupiter's strong magnetic field creates an electrical connection, a current that flows from Io to Jupiter. That part is certainly electrical.

Only a few supernovae have been successfully matched with pre-observed stars, so i son't think either question can be directly answered. The mass loss for Eta Carinae, and extreme supermassive star, is about 0.001 solar masses per year, which is the highest mass loss rate I know of. Wolf Rayet stars blow off from 0.00001 to 0.0001 solar masses per year.

As for age, stars go supernova because of their mass, not their age. For single stars and type II (core collapse) supernovae, I should think they are mostly less than a few hundred million years old, and certainly not a billion. But in multiple star systems, and type I supernovae, it's harder to tell. The more massive star might evolve to a white dwarf, then accrete material from the other star, before it goes boom. then it depends on the accretion rate, and I don't know much about that.

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell