What you're seeing is one of the endgame animations.
Which is curious, for it's possible to defeat Astra, I have done it by using fully upgraded fighters and all-elite pilots. But this is only possible if you don't move the mothership close, whichtriggers the animation and ends the game.
Even if you do manage to defeat Astra, however, the game ends - if you move close to the station with the mothership afterwards, SW2 simply crashes to desktop. Guess the designers didn't think the Super Chimera could be destroyed?
