FU Orionis Star

In stellar evolution, an FU Orionis star (also FU Orionis object, or FUor) is a pre–main sequence star which displays 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. These stars are named after their type-star, FU Orionis.

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 around 10−4 solar masses per 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 Orionis, V1057 Cygni, V1515 Cygni, and the recent addition the embedded protostar V1647 Orionis, which erupted in 2004 January.