Looking for the book where a space ship made from a giant atomic atom journeys to the center of the galaxy only to find all the stars had gone supernova.
This sounds like the short story "At the Core" by Larry Niven.
Four years after the events in the other short story "Neutron Star", spaceship pilot Beowulf Shaeffer is on Jinx, a planet orbiting Sirius B, when he is again contacted by the Puppeteers, this time by the Regional President of General Products on Jinx, who offers him a chance to guide a cramped (but very fast) experimental ship to the center of the galaxy as a promotional stunt. Shaeffer is offered one hundred thousand stars to make the trip, plus fifty thousand stars to write about it; he is also given the rights to sell the story. Shaeffer, seeing the value of such a promotion (as well as the value of his pay) agrees to go, naming the ship Long Shot.
Finally, Shaeffer halts, unwilling to go on. He contacts the Puppeteer via hyperphone and tells him that the galaxy is exploding: the Core stars have gone supernova and are causing other stars to do the same as the impinging radiation heats them up. The Puppeteer asks if Shaeffer wishes to be released from his contract without pay and Shaeffer declines, amused. The Puppeteer explains that if Shaeffer does not enter the Core, he forfeits on the contract. Shaeffer explains that he will show the radiation readings and brightness levels, and if questioned under truth drugs, any court in Known Space would know the Puppeteers tried to get Shaeffer to fly into the center of the holocaust, and it would rule against the Puppeteers. The Puppeteer concedes, and tells Shaeffer to return home.