From publisher blurb: The Necklace is the thickest part of a gas torus around a neutron star, itself a part of a close binary with a hot F-type star. The gas torus was formed when another gas giant orbiting the neutron star was slowly stripped of its enormous atmosphere. The gasses stayed in a ring, slowly orbiting the neutron star. The torus changed as life spread into it. Life begets oxygen, and oxygen begets life. The Necklace is the inner, denser part of the gas torus, which is breathable for normal humans and earth animals. There is no hard and fast boundary - the air just gradually gets thinner and thinner. This area is about 20 thousand kilometers in diameter. The entire torus has a thickness diameter of about 2 million kilometers, and an overall diameter of 9 million kilometers. There is room for a lot of adventures among tree-covered asteroids, and along an artificial River a billion kilometers in diameter, with half sunk asteroids in the river as islands, and all teeming with life.