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The formation of stars and their planetary systems is among the topics of greatest interest in modern astronomy. Star formation in the Milky Way galaxy is an ongoing phenomenon, and with modern astronomical instrumentation we can observe the process in many locations and in various states of evolution. Star formation occurs in the filamentary structures observed in the interstellar medium. In the densest strands, the gas that constitutes the filaments becomes unstable and forms clumps of material bound together by gravity. If dense enough, these collapsed blobs of gas eventually go on to become newborn stars. Rotation causes the development of a circum-nuclear disc, and an outflow of gas occurs along the symmetry axis of the disc. The outflow begins to open its way through the infall material, and after about a million years the rate of infall decreases significantly. A proto-star with a well-established outflow, a weaker infall, and a disc, is formed. Eventually, the accretion ceases, the outflow weakens and the disc is largely eroded, although a remnant planet-forming disc remains in orbit around the star. The temperature in the disc decreases steadily from the proto-star, and determines the chemical composition of the planets.

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