What does Montag see in the sky that he thinks is a bomb?

Without knowing which section you are referring to, these quotes both allude to the way in which Montag sees the stars in the sky.

The way the clouds moved aside and came back, and the way the stars looked, a million of them swimming between the clouds, like the enemy discs, and the feeling that the sky might fall upon the city and turn it to chalk dust, and the moon go up in red fire; that was how the night felt.

He saw a great juggernaut of stars form in the sky and threaten to roll over and crush him.