As others have said, I think the lack of detail was deliberate. I doubt Black Industries even ever intended to finally reveal what the Tyrant Star was, in order to 1) leave it up to the GM of any given campaign and 2) because so often when you solidify these kind of ideas, they are never as awesome as people might have thought. I certainly felt this with the Deathwatch mystery which turned out to be "Lol, Necrons are hiding inside stars!" That is unless the Haarlock Legacy trilogy is meant to solve it somehow, but then the ending for that was a bit lame (see point 2) and didn't explain anything anyway, and so I hope not.
Myself, I don't even think players were ever meant to solve the riddle of the Tyrant Star. It is probably mostly intended to be a background thing, which players maybe encountered, such as if it made an appearance at a planet and the usual result of a the planet descending into chaos resulted, or be obviously connected to a secret, but not necessarily what the players are directly looking into (see the Haarlock legacy again). An unknowable, unstoppable force to give flavour to the setting.