Eternal Light — Paul McAuley

In the aftermath of an interstellar war an enigmatic star is discovered, travelling towards the Solar System from the galactic core. Its appearance adds a new and dangerous factor in the turbulent politics of the inhabited worlds as the rival factions - the power-holders of the ReUnited Nations, the rebels who secretly oppose their power, and the Religious Witnesses - all see advantages to be gained.

But what awesome technology started the star on its journey half a million years ago - and why?