Captain Marvel Jr #76 (1949)

Freddy Freeman is peddling papers when Ajax Dudd, crackpot scientist, offers that he's finished his new Heat Machine. He has the same temperament as Dr. Sivana (wanting to be King of Earth,) but has none of the capability. He threatens that all human life would die if he jacked the temperature above 130F. Freddy thinks aloud in Dudd's junkyard how none of these machines work and soon returns to his Newsstand. He finds that it's become hot enough to remove his jacket and he figures it's likely over 100F now. He hopes it'll get cooler by nightfall, but it doesn't. Instead, it just gets hotter until it's a deadly 120 out, causing a dog to go mad and attack a woman. Freddy calls CAPTAIN MARVEL to become Captain Marvel, Jr. and drags the dog to the pound, finding many more mad dogs on the way. Soon, a radiator busts and nearly scalds a child to death standing on the street nearby, but Junior guards him with his cape. He finds a man suffering from dreaded heat syncope and moves him into the shade so he can recover somewhat, but notes that it won't help from the inherent heat in the air. Junior soon realizes that Ajax Dudd did create a Heat Machine that works (he's just sitting on a large ice cube to counteract it.) Junior smashes it and finds that somehow destroying it only made things even hotter, discovering the Heat Machine never worked the whole time and the Sun itself is the real issue. Consulting the Palomar Observatory, he finds that their solar telescope indicates that the Sun is getting hotter! So, he dives into the heart of the sun and finds there's a city there and that the telepathic Sun People there have ratcheted the heat up to 25,000,000 to be more comfortable. They notice that it's a million degrees hotter in the Sun that day, something they don't notice themselves, though Junior points out it's killing the world. He soon finds there's a Sun War between rival Sun Peoples with billion degree heat rays and that neither side is willing to stop to let the Earth live. Junior realizes that this will cause the Sun to go nova, eradicating all life on the Sun too. Junior shows them this by creating a tiny nova that terrifies the Sun People into signing a peace treaty. The United Sun lives on as Captain Marvel, Jr. throws the nova rock into space and hard enough to enter a different Solar System. Returning to Earth, stray dogs are now friendly. Later, Freddy discovers that the heat somehow made Ajax Dudd sane and he now makes helpful inventions.