FU Orionis

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In stellar evolution, FU Orionis objects (or FUors) are pre-main sequence stars which display an extreme change in magnitude and spectral type. One example is the star V1057 Cyg, which became 6 magnitudes brighter and went from spectral type dKe to F supergiant.

The current model associates the FU Orionis flare with abrupt mass transfer from an accretion disc onto a young, low mass T Tauri star. Mass accretion rates for these objects are estimated to be:

$\dot{M} \approx 10^{-4} \frac{M_\bigodot}{year}$

The rise time of these eruptions is typically ~ 1 year, but can be much longer. The lifetime of this high accretion, high luminosity phase is on the order of decades. However, even with such a relatively short timespan, an FU Orionis object has yet to be observed shutting off. By comparing the number of Fuor outbursts to the rate of star formation in the solar neighborhood, it is estimated that the average young star undergoes approximately 10-20 Fuor eruptions over its lifetime.

The prototypes of this class are: FU Ori, V1057 Cyg and V1515 Cyg; being the more recent addition the embedded protostar V1647 Ori, which erupted in 2004 January.