Fuel was created by the explosion of supernovae, still solar but not our sun.

By that logic, all of the Earth and the moon were once parts of stars, so tidal and geothermal are also solar.

When people say "solar energy", they are usually referring to first order solar energy, directly from photons, not second or third order solar energy after it has been trapped into other sources of potential energy.

If it's not our sun then it isn't solar.

Pedantry I can get behind. Cheers

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Probably neutron star collisions, actually.