Harry can talk to LV about the life cycles of stars and the heat death of the universe. All this could force LV to rethink what it means to be immortal when the sun engulfs the earth or the universe hits maximum entropy. This could buy some time.
I agree that this would be relevant, but Harry doesn't know the literal text of the prophesy yet. Only discussion of "destroying the world" has happened in his presence, not "tear apart the stars".
The fact that "protecting the earth" in the very long run requires protecting the earth from solar flares and supernovas hasn't yet been understood by Voldemort.
Harry can talk to LV about the life cycles of stars and the heat death of the universe. All this could force LV to rethink what it means to be immortal when the sun engulfs the earth or the universe hits maximum entropy. This could buy some time.