FU Orionis stars

FU Orionis outbursts the brightening of an FU Orionis star. Such outbursts are thought to occur when the mass accretion rate through the accretion disk increases by orders of magnitude. 

FU Orionis star (FU Ori-type star or Fuor) a pre-main-sequence star that recently underwent an extreme brightening at visual wavelengths, typically of five magnitudes or more. They are named after their prototype FU Orionis. These stars rapidly brighten, then remain almost steady or slowly decline by a magnitude or two over a period of decades. It is proposed that the brightening is driven by a runaway disk accretion, a result of an instability. 

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