James Blish 
Warriors of Day 

Tipton Bond was a man who lived for challenges – and Earth had no more challenges for him. But on Xota: One of the giants’ faces bent over him, sardonically benign. "You are an Earthman, little human electron. You are a denizen of a planet which the Warriors of Day will reach and engulf, in a bare thousand years. Did you think to escape us by coming to Xota? You have brought yourself closer to the thing which you thought to escape. You will share in the general holocaust when the Wild Star arrives!" Giants they might be, Bond decided, but they had limitations. "What is this Wild Star?" he called. The slow thunder answered: "It is the most condensed of white dwarfs, almost pure neutronium throughout. And what makes a sun a runaway sun? The answer, little one, is very simple: The Warriors of Day!"