The Xeelee Sequence (; ZEE-lee) is a series of hard science fiction space opera novels, novellas, and short stories written by British science fiction author Stephen Baxter. The series spans billions of years of fictional history, centering on humanity's future expansion into the universe, its intergalactic war with an enigmatic and supremely powerful Kardashev Type IV alien civilization called the Xeelee, and the Xeelee's own cosmos-spanning war with dark matter entities called Photino Birds. The series features many other species and civilizations that play a prominent role, including the Squeem (a species of group-mind aquatics), the Qax (beings whose biology is based on the complex interactions of convection cells), and the Silver Ghosts (symbiotic organisms encased in reflective shells). Several stories in the Sequence also deal with humans and posthumans living in extreme conditions, such as at the heart of a neutron star (Flux), in a separate universe with considerably stronger gravity (Raft), and within eusocial hive societies (Coalescent).The Xeelee Sequence deals with many concepts stemming from the fringe of theoretical physics and futurology, such as exotic-matter physics, naked singularities, closed timelike curves, multiple universes, hyperadvanced computing and artificial intelligence, faster-than-light travel, and the upper echelons of the Kardashev scale. Thematically, the series deals heavily with certain existential and social philosophical issues, such as striving for survival and relevance in a harsh and unknowable universe and the effects of war and militarism on society.As of August 2018, the series is composed of 9 novels and 53 short pieces (short stories and novellas, with most collected in 3 anthologies), all of which fit into a fictional timeline stretching from the Big Bang's singularity of the past to the eventual heat death of the universe and Timelike Infinity's singularity of the future. An omnibus edition of the first four Xeelee novels (Raft, Timelike Infinity, Flux, and Ring), entitled Xeelee: An Omnibus, was released in January 2010. In August 2016, the entire series of all novels and stories (up to that date) was released as one volume in e-book format entitled Xeelee Sequence: The Complete Series. Baxter's Destiny's Children series is part of the Xeelee Sequence.
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