Supernovas produce a shockwave in the act of throwing most of the star's mass. This shockwave would have shoved the Mu Relay (located at the edge of the cleared out portion of a solar system) before it would have been hit by the really destructive part of the explosion.It's the theory that makes the most sense if you also take into account the Alpha Relay from the ME2: Arrival DLC. The asteroid there was capable of delivering 384.7 gigatons of kinetic energy. But considering how fast the relay fell apart upon contact, it took only a minute percentage of the impact. So, you're looking at single-digit gigaton to triple-digit megaton range; magnitudes short of a supernova.