It was great to see the Supernova happen on the last hyperspace jump of the stream, perfect timing
I’ve been doing a little further searching for mission trigger info - some interesting stuff from Ersh on the World Of Spectrum / Spectrum Computing forums who has used a debugger on the game - they reckon:
Mission 1 (Sun going super nova) is triggered after you've made 32 hyperspace jumps in any galaxy but the first.
Mission 2 (The cloaked Asp) is triggered after you've made 128 jumps in any galaxy but the first.
Mission 3 (Thargoid attack) is triggered after you've made 254 jumps in any galaxy but the first
Looks like I’ll be getting very familiar with my current Galaxy chart

Edit: it also ties in with another snippet of info that said all the missions after the first one can be completed in Galaxy 3.
My current plan of action: find two systems within 1ly of each other and hyperspace ping-pong between them until I need to refuel - I’ll report back with findings!
Also, I seem to be unusually lucky with the
Ship Boarded By Pirates game over screen, going by the percentages they discovered:
0.39% likelihood in Corporate States and Democracies
0.78% likelihood in Confederacies and Communist States
1.17% likelihood in Dictatorships and Multi-Governments
1.56% likelihood in Feudal Worlds and Anarchies