a race locks up their entire existence into a large crystal, which is an enormous solid-state holographic memory.
…desparate last attempt to preserve themselves. sun going nova in a thousand years, sun is isolated in inter-galactic space; far from any other stars. energy will run out. they engineer their planet into a spacecraft, to make the eons-long journey to other star systems. some don’t believe that their sun will nova; fight; lose. the planet is reconstructed; ruining the biosphere; but the crystal preserves the memory, in incredible detail.
The planet/ship starts its long journey; there is a voluntary reduction in reproduction; the population is small; memories of the individuals also preserved in the crystal. The remaining reproducing survivors are to man the planetship, but self-destruct because of lingering insanity/group suicide/0 reproduction/conflicts. the very last survivors lock their last memories in the enormous memory banks, set things to run automatically as best they can (which is very good), and die. The ship continues its lonely journey, with its incredible cargo of the most precious treasures of a lost civilization.
its parent star went nova, as predicted; the outer surface was burned from that, but it had gained enough distance in time to survive; the mechanisms deep inside the planet continued; the few external sensing components were mostly OK. Occasional repair bots would make the journey to the surface. After a billion years the ship has a fair start on it’s journey through the cold.