It is important to remember what has just happened to Guy Montag before he realises that his wife, Mildred, has taken an overdose. Note how his conversation with Clarisse has made him aware of a real emptiness within himself and the fact that he is not happy. In addition, the realisation that his wife has taken an overdose and tried to kill herself produces an intense emotional reaction in Guy Montag, which is mirrored uniquely by the jets screaming across the sky. Note how these jets are described:
As he stood there the sky over the house screamed. There was a tremendous ripping sound as if two giant hands had torn ten thousand miles of black lines down the seam. Montag was cut in half. He felt his chest chopped down and split apart.
These jet bombers "did all the screaming for him" and the sound of the bombers allows him to express his emotional desolation through his scream but also the feeling of being chopped in two. However, note how his belief about the stars likewise indicates this as well:
He felt that the stars had been pulverised by the sound of the black jets and that in the morning the earth would be covered with their dust like a strange snow.
Again, having just been outside at night and having just looked at the stars, this belief in their disappearance is highly indicative of Montag's new state. It is not only caused by his wife's suicide attempt, but also by the new-found desolation that he is aware of in himself. For a man who burns books for a living, there is an irony in his fear that the "ashes" of the stars will cover the earth.