I had asked why the Babyeaters and Superhappies have not intentionally created novae. But now I think it's pretty likely that the Babyeaters actually caused the nova. The Babyeaters were in the system first, despite being the least technologically advanced race, and despite having made special preparations for the hostile environment (the mirror shielding). If they had come in response to the nova, they probably would have been the last to arrive.