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A FU Orionis star (or FU Orionis object or FU Ori star or abbreviated FUor) is a type of variable star that shows large changes in magnitude over fairly long periods, e.g., growing brighter over the course of some year and remaining bright for as long as a decade before dimming again. Current interpretation is a pre-main-sequence star (PMS), the transients stemming from periods of accretion from a protoplanetary disk. Observations have shown signs suggesting disk instability. The name of this class of stars stems from the first such star noted, FU Orionis (aka FU Ori).