Triggers:FUOri

FU Orionis (V1647 Ori) is young pre-main sequence stars, illuminating a McNeil's nebula in the vicinity of NGC 2068 star-forming region.

Plot from D.Chochol et al. 2006.

Characteristics:

• Very rare
• Outbursts repeat with a time scale of 40 years (1978, 2003)
• Amplitude: 5 mag over 4 months
• Outbursts last for 2 years
• Spectrum: red, heavily veiled continuum with strong emission of <math>H_{\alpha}</math>; in blue consistent with an early B spectral type
• X-ray variability present

More general characteristics of FUors, based on other examples

• FU Ori and V1057 Cyg rise over 1 yr, whereas V1515 Cyg rise over 20 years
• FU Ori and V1515 Cyg decline over 20-100 years, V1015 Cyg decays faster (10 yrs)
• Spectra: F or G supergiants
• Some of them are embedded in nebulas
• OO Serpentis is newly discovered another example of FU Ori Kospal et al. 2007.