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Protoplanetary DiscA protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. They differ from the discs surrounding the primary components close binary systems in their size and temperature. Protoplantary discs have radii up to 1000 Astronomical Units and are rather cool. Only their innermost parts reach temperatures above 1000 Kelvin. They are very often accompanied by jets. Protostars typically form from molecular clouds consisting primarily of molecular hydrogen. When a molecular cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity. As the cloud shrinks, conservation of angular momentum causes the random motions originally present in the cloud to become one coherent rotation. This rotation causes the cloud to flatten out (much like forming a flat pizza out of dough) and take the form of a disk. The initial collapse takes about 100,000 years. After that time the star reaches a surface temperature similar to that of a main sequence star of the same mass and becomes visible. It is now a T Tauri star. Disc accretion continues for another 10 million years, before the disc disappears, which means that it is either really gone or just does not emit radiation any more because accretion has ceased. Protoplanetary discs are thought to be the progenitors of planetary systems. Gravitational interactions may cause the dust and gas in the disk to condense into planetesimals. This process competes against the stellar wind, which drives the gas out of the system, and accretion, which pulls material into the central T Tauri star. See also:
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