If Wolverine went into the sun, that wildcat is really lucky because everything should've melted except maybe his adamantium coated bones. His T-cells and regenerative cells should've died. That would teach other mutants with healing factors (like Sabertooth) not to abuse their gifts...Anyway Superman can probably stand in the hottest sun for a while but I'd bet it would feel uncomfortable to him. Like "Kal-el" said earlier, Superman's powers have indeed been limited. I mean how challenging would Doomsday for example be against a purely invulnerable person?...That's why they say he's "virtually" invulnerable. Different comics explain Superman's powers in different ways. How many of you read Birthright"? Superman's body, by harnessing solar radiation became stronger than titanium but is common in appearance. Other sources say he's protected by an aura that's activated by harnessed solar radiation...It also protects his suit to a certain degree. Supes drains solar radiation but doesn't get stronger from the heat. Besides Supes would probably become soup if he went to Krypton's sun while it was exploding.