Start with, say, a square mile of construction material. I made educated guesses about its surface density. Suppose it gets from the nova and surrounding stars about what the Earth receives from the Sun — something over a thousand watts a square yard. Assume total efficiency of conversion: mass equals energy over cee squared.
That gave it a doubling time of fifteen years. I dreamed of numbers: one, two, four, eight, sixteen... It was already too big to handle. It would be the size of the Earth after a couple of centuries, the size of Sol a little later.
Give it a thousand years and you could wrap up the Galaxy like a birthday present. Doubling series grow fast. And no one knew how to cut Xeelee construction material.
-Vacuum Diagrams, "The Xeelee Flower"