Raft - Stephen Baxter

A spaceship from Earth accidentally survives the transition to a radically different universe. Five-hundred-years later, their descendants still struggle for existence.

The Miners live on the Belt, a ramshackle ring of dwellings orbiting the core of a dead star. They trade for food from the Raft, a structure built from the wreckage of the ship, and where a small group of scientists preserve the ancient knowledge which makes survival possible. Rees is a Miner whose curiosity about his world makes him a stowaway on a flying tree.

And what he finds will change his world...