dying world

The Universe Wreckers – A serial novel by Edmond Hamilton about the attempt by the beings of Neptune to explode the Sun into a double star which would destroy all of the inner planets of the Solar System. Men of Earth must fight to save the Earth from utter immolation.

Against The Fall Of Night by Arthur C. Clarke
The city has existed for billions of years. Surrounded by impenetrable walls, the inhabitants, the last of mankind, are immortal. Every need and desire is furnished by robots. No one has ever left the city. But one young man wants to know what else is out there. . . .