Couldn't Goku just teleport Superman to the middle of nowhere using instant transmission, and then leave him there? Without the sun Superman would be screwed, right?
Correct, that probably could work, but there are a couple of limiting factors that make it highly unlikely:
First, Goku is dumb. I'm sorry, but he is. He's a brilliant martial-artist to be sure, and a decent tactician as well, but he's always so focused on fighting that he doesn't recognize outside factors (like Superman's connection to the sun, for example)... at least, not quickly enough to strategize around it. Having no formal education, Goku likely wouldn't have thought of a complex plan like this, and wouldn't know outer space well enough to transport to an empty sector, even with ki senses.
Second, this tactic requires Goku to contact Supes, maintain contact and concentration long enough to teleport them both (which is only a fractional second, but between these two that's practically an
entire season eternity), then break away from Superman, maintain concentration under the enormous stresses of exposure to deep space and god-level combat, and keep that distance long enough to teleport himself back alone. Highly unlikely, unless Goku can prevent Superman from figuring out what he's doing, and even then the chances are slim. (If this were an RP campaign, that would be a LOT of dice rolls and negative modifiers.)
Third, Superman does know outer space. Unlike Goku, Superman navigates deep space on his own fairly regularly; he knows his way around and how to survive there. Even if Goku was able to pull off this trick, Superman would likely have enough time to get to a usable star or planet before his energy ran out.
Their sun was yellow once, right?
Yes, it was. The Kryptonians were once human-like in their physiology, with the exception of "standard" high-gravity musculature. As their sun and planet aged and became less hospitable over who-knows-how-many millenia, so too did the species evolve stronger anatomy to survive it. They couldn't have conquered the galaxy when their sun was yellow because they didn't gain godly superpowers from it at the time. Maybe that was retconned in Man of Steel, possibly earlier, but I'm going by the original Superman movies and animated series.