Symbolism, Imagery, Allegory
Merlin can tell a lot about the future by reading the stars. Okay, actually, the stars merely confirm what he already knows to be true by other supernatural means. That's because stars in this book are emblems—they represent a character or a nation. As Merlin returns from Ireland, for example, he has a vision of his father's deathbed. When he lands, he sees a star in the sky that seals the King's fate:
The king-star rose again that night, looking, men said, like a fiery dragon, and trailing a cloud of lesser stars like smoke. But it did not need an omen to tell me what I had known since that night on the crest of Killare, when I had vowed to carry the stone from Ireland, and lay it upon his grave. (IV.10.45)
Yep, that's a star being snuffed out by the universe. It works as a sign because there's a big equal sign drawn between the star and Ambrosius. We could say that the king-star represents Ambrosius' body.
But the king-star doesn't just belong to Ambrosius. It belongs to the King—whoever that may be. On the night that Arthur is conceived in Tintagel, a half-dead Merlin is cheered up by the sight of another king-star—and this one is in much better shape:
Then suddenly, directly overhead, the clouds parted, and there, sailing through them like a ship through running waves, the star.
It hung there among the dazzle of smaller stars, flickering at first, then pulsing, growing, bursting with light and all the colours that you see in dancing water. I watched it wax and flame and break open in light… (V.8.22-23)
That star stands for both the body and reign of the new and improved king. Merlin is completely chuffed by the sight of this white-hot baby star because he knows that his visions have been right. And that means that the great protector of Britain is about to step onto the stage.