Cold be hand and heart and bone,
And cold be sleep under stone:
Never more to wake on stony bed,
Never, till the Sun fails and the Moon is dead.
In the black wind the stars shall die,
And still on gold here let them lie,
Till the dark lord lifts his hand
Over dead sea and withered land.
In The Silmarillion, the Valar (gods) create two lamps to provide light upon the world, and upon their destruction by Melkor (Sauron’s master), they grow two trees. These are destroyed by Ungoliant (the mother of Shelob) at Melkor’s behest, but their last flower and fruit are made into the sun and moon. Their light was captured in the silmarils, one of which resides in the sky as the northern star. 
 Thus, to destroy the sun, moon, and stars is to complete the task Morgoth (as Melkor came to be known) sought to accomplish at the beginning of the First Age. This is predicted to announce the beginning of Dagor Dagorath (the Tolkien Ragnarok/Armageddon), as Morgoth shall break free from imprisonment and wreak havoc. 
