What happens when Superman goes near other types of stars like white dwarfs?
In the olden days, non-G-type stars not only could not be used as a source of fuel for Superman's powers, but actively sucked the powers out of him (e.g., the classic Eliot S! Maggin story, "Must There be a Superman?"). Because DC comics is presently run by monkeys, this has been in part revived (Superboy Prime was originally caged in a red sun eater, for example).*
Leaving aside the idea that Kryptonians evolved superpowers fueled by the light of another kind of star than the one they lived under, presumably no one who ever wrote a Superman comic ever looked up the term "blackbody radiation."
*You will note that I cut Maggin slack and Johns none. Historical relativism is an acceptable method of dealing with the world.
Anyway, Kryptonians capable of serious interplanetary travel would rapidly discover the effect of yellow suns on their biology, and would basically be the sole arbiter of the Alpha Quadrant's fate. The only people who could give them a run for their money would be our Assorted Space Gods (Qs, Founders, Organians, Metrons, the Greek xenotheoi like Apollo, and so forth).
Are we also assuming Krypton never exploded? Because that's the source of kryptonite.