Flux by Stephen Baxter

Flux

As the novel begins, a glitch - an instability of the magnetic field inside the star caused by changes in the star's rotation - is about to destroy a net made up of ropes, where a group of 50 humans live. During this several of the older humans are killed, and importantly the humans lose their primary food source - a herd of "air pigs" - animals indigenous to the star.

In order to find more food, Dura, together with her young brother Farr, Adda (the oldest of the Humans and one of the novel's main characters), Philas (wife of a man killed during the glitch, this man was also seeing Dura) and 6 other adults travels high into the top of the mantle of the star to find food in the forest.

Whilst there Adda is injured by a pregnant sow air pig. Just after, the humans encounter Toba Minaxx, a human from Parz City. Parz is a massive wooden city where other star humans live, with a functioning economy and upper and lower classes etc. As becomes apparent, the ancestors of Dura's group did originally live in Parz, but left when their belief that the Xeelee should ultimately be accepted as being for the good of humanity was not accepted by the rulers of Parz. A hospital, "The Hospital of the Common Good" in the heart of Parz City is Adda's only chance for survival.

While in Parz, we meet several other characters: Muub, the head physician and advisor to Hork, the administrator of Parz. In order to pay for Adda's treatment, Dura's labour is sold to a "mantle farm" (where trees are harvested for use as fuel or as building blocks for the city), and Farr is sold to work in the underbelly of the city.

Farr makes two friends whilst here: Toba Minaxx's son Criss and Byza, a fellow miner. Criss teaches Farr to board (using a specially constructed plate to "surf" along the flux lines), an ability which allows him to escape from the eventual attack by the Xeelee.

After various plot points, the characters realise the instabilities are actually being caused by the attack of the Xeelee, and the next instability could destroy both Parz and possibly the star itself. Hork calls a combination of Muub, Dura (called due to her experience as a star human), Adda, Farr and a scientist to go down into the inhabitable centre of the star and try and retrieve ancient weapons, supposedly left by the humans who originally created the star human race.