Raft by Stephen Baxter

The provided text details a post-apocalyptic future where humanity survives in a Nebula, a vast, gas-filled environment. Life is sustained on two primary settlements: the Raft, a more technologically advanced floating city, and the Belt, a harsh mining colony orbiting a collapsed star kernel. The narrative follows Rees, a miner from the Belt, as he uncovers the truth about their origins as descendants of a lost Ship and the Nebula's slow decay. This realization propels him to join the Scientists on the Raft, advocating for a perilous journey through the Core of the Nebula to find a new, viable home, ultimately leading to the launch of the Bridge, a section of the Raft repurposed as a ship, carrying a select few towards an unknown future.