Explain the line "they slowly grew into millions," in "The Ballad of Father Gilligan."
Here's the context of the line you're having trouble with, from "The Ballad of Father Gilligan," by Yeats:
He knelt, and leaning on the chair
He prayed and fell asleep;
And the moth-hour went from the fields,
And stars began to peep.
They slowly into millions grew,
And leaves shook in the wind;
And God covered the world with shade,
And whispered to mankind.
The stars, as the sun goes down and the moon rises, as it gets darker, turn into millions. These millions reflect the majesty of God, as later, He who commands the planets, as well as these stars, takes pity of the poor priest asleep in the chair.
The stars begin as a peep, but, again, as darkness covers the world and the leaves are blown by the wind, the stars become visible and turn into millions.
If you want to know what this line means, you just have to look in the poem to see what comes before it. You have to look before the line you cite to see what it is referring to.
In this case, the line is referring to the stars. It is the stars that grow into millions.
What this means is that it is getting to be late at night. As it gets later in the night, the sky gets darker and darker. As it does that, you can see more stars and they seem to "grow into millions."
Through it all, Father Gilligan is sleeping...