Poul Anderson 
Enemy Stars 

They built a ship called the Southern Cross and launched her to Alpha Crucis. Centuries passed, civilisations rose and fell, the very races of mankind changed, and still the ship fell on her headlong journey toward the distant star.After ten generations the Southern Cross was the farthest thing from Earth of any human work — but she was still not halfway to her goal.Here is an absolutely absorbingly exciting tale of the far future from one of the giants in the field of Science Fiction writing.