FU Orionis

An FU Orionis outburst is a dramatic, months-long brightening event observed in certain young, pre-main-sequence stars. The star's luminosity can increase by several magnitudes and remain elevated for years or decades. The outbursts are believed to be caused by sudden increases in the rate of accretion from the star's surrounding disk.

FU Orionis underwent its famous brightening event in 1936, when it flared from a faint star to one visible in modest telescopes. This was the first recorded instance of the phenomenon that now bears its name. The star has remained brighter than its pre-outburst state ever since.

The leading theory is that thermal or gravitational instabilities in the star's protoplanetary disk cause a sudden surge of material falling onto the star, dramatically increasing its brightness. This accretion-driven process is fundamentally different from other types of stellar variability. The exact trigger mechanism is still studied.

FU Orionis is located in the constellation Orion, within the Orion Nebula star-forming region at a distance of roughly 400 to 500 light-years from Earth. It is situated in one of the nearest active stellar nurseries to the Sun. This proximity has made it one of the best-studied young variable stars.