Haven’t played the DLC either yet, not sure if that adds anything, but:

[Spoilers for mid-game]

!While you play, you’ll encounter clues that point towards the sun station being the reason for the sun dying, as it was built to make the sun go supernova and use the resulting energy to find the eye of the universe.!<

[Spoilers for late- to endgame]

!Later on, it turns out that the sun station is actually not the reason, as it just didn’t work as the Nomai planned. The sun is actually going supernova because the universe is dying as a whole, as you can actually see all around you, with loads of supernovae going off in the sky. Now sadly, there’s no explanation given for why that is happening much earlier than everyone expected, iirc. I guess that’s a final mystery that’s indeed left unanswered.!<