Warning: contains spoilers for Action Comics #1037!
DC's Superman is considered one of the strongest heroes in comics, able to fly faster, punch harder, and survive more than most of his compatriots - but even he has his limits. The Man of Steel is known for fighting to protect Earth and the vast reaches of space, but it's the latter that proves to be more difficult in light of his little-known weakness. Unfortunately for Superman, Action Comics #1037 depicts a villain who knows exactly how to defeat Superman: the vicious Mongul of Warworld.
When he was first introduced in 1938, Superman had his usual grab-bag of powers, but they were noticeably toned down from the current modern age. He could leap tall buildings, but he couldn't fly; he could bend iron bars, but couldn't punch holes in mountains; he was bulletproof, but "...nothing short of a bursting shell can pierce his skin!" Today, Superman is so extraordinarily powerful that his most dangerous enemies almost always have a supply of Kryptonite - Superman's classic weakness - on hand. But Kryptonite is not the only inherent weakness of the Man of Steel.
In a battle between Mongul and Superman, Mongul is surprisingly winning (mostly because Superman has been kept in the fight via his magical allies). Mongul reveals his teacher informed him that Clark Kent's strength came from a yellow sun affecting his Kryptonian cells differently than a red sun (a red giant was the local star of Krypton, and thus the Kryptonians had no powers while on their homeworld). "Beneath our feet are the star forges," explains Mongul as he attacks Superman. "Generators the size of warships that produce radiation...like that of a red sun."
Mongul's gambit means Superman has no power on his world...or any planet that orbits a red sun. This is perhaps even more threatening of a weakness to Superman than Kryptonite; while radioactive pieces of Superman's homeworld of Krypton are of course finite, the universe has plenty of systems with red giant stars (Gacrux, the nearest red giant star in the real world, is only 88.6 lightyears away from Earth). Any supervillain who wants to enact an evil plan without interference from Superman need only set up shop on a planet orbiting a red giant, and Superman poses no more threat than that of an ordinary human.
Superman has been known to power through the effects of Kryptonite on occasion, but while red solar radiation won't kill him, it essentially turns him into a mortal human like Batman, with all the inherent weaknesses of the species. This turns Superman from a nigh-unbeatable threat into someone who could lose against a trained fighter, provided they choose the location close to a red giant. Since Mongul's teacher knows this weakness - and presumably learned it from someone else - this makes Superman much easier to defeat in the far reaches of space.