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-type supergiant. These stars are named after their type-star, FU Orionis.

The current model developed primarily by Lee Hartmann and Scott Jay Kenyon associates the FU Orionis flare with abrupt mass transfer from an accretion disc onto a young, low mass T Tauri star.[1][2] 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 on the order of 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, as of 2015 no FU Orionis object had been 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 spectrum of FU Orionis stars are dominated by absorption features produced in the inner accretion disc. The spectrum of the inner part produce an spectrum of a F-G supergiant, while the outer parts and slightly colder parts of the disk produce a K-M type supergiant spectrum that can be observed in the near-infrared. In FU Orionis stars the disk radiation dominates, which can be used to study the inner parts of the disk.[3]

The prototypes of this class are: FU Orionis, V1057 Cygni, V1515 Cygni,[4] and the embedded protostar V1647 Orionis,[5] which erupted in January 2004.
See also

Orion variable
T Tauri star


Bertout, C. (1989). "T Tauri Stars-Wild as Dust". Annu. Rev. Astron. Astrophys. 27: 351. Bibcode:1989ARA&A..27..351B. doi:10.1146/annurev.aa.27.090189.002031.
Reipurth, B. (1990), "FU Orionis eruptions and early stellar evolution", Flare Stars in Star Clusters, 137: 229, Bibcode:1990IAUS..137..229R
Siwak, Michał; Winiarski, Maciej; Ogłoza, Waldemar; Dróżdż, Marek; Zoła, Stanisław; Moffat, Anthony F. J.; Stachowski, Grzegorz; Rucinski, Slavek M.; Cameron, Chris; Matthews, Jaymie M.; Weiss, Werner W. (October 2018). "Insights into the inner regions of the FU Orionis disc". A&A. 618: A79. arXiv:1807.09134. Bibcode:2018A&A...618A..79S. doi:10.1051/0004-6361/201833401. ISSN 0004-6361.
"V* V1515 Cyg". SIMBAD. Centre de données astronomiques de Strasbourg.

"V* V1647 Ori". SIMBAD. Centre de données astronomiques de Strasbourg.

Juhan Frank, Andrew King, Derek Raine (2002). Accretion power in astrophysics, Third Edition, Cambridge University Press. ISBN 0-521-62957-8.


Variable stars
Cepheids and

Type I (Classical cepheids, Delta Scuti) Type II (BL Herculis, W Virginis, RV Tauri) RR Lyrae Rapidly oscillating Ap SX Phoenicis

Blue-white with
early spectra

Alpha Cygni Beta Cephei Slowly pulsating B-type PV Telescopii Blue large-amplitude pulsator


Mira Semiregular Slow irregular


Gamma Doradus Solar-like oscillations White dwarf

Protostar and PMS

Herbig Ae/Be Orion
FU Orionis T Tauri

Giants and

Luminous blue variable R Coronae Borealis (DY Persei) Yellow hypergiant

Eruptive binary

Double periodic FS Canis Majoris RS Canum Venaticorum


Flare Gamma Cassiopeiae Lambda Eridani Wolf–Rayet


AM Canum Venaticorum Dwarf nova Luminous red nova Nova Polar Intermediate polar Supernova
Hypernova SW Sextantis Symbiotic
Symbiotic nova Z Andromedae


Rotating ellipsoidal

Stellar spots

BY Draconis FK Comae Berenices

Magnetic fields

Alpha² Canum Venaticorum Pulsar SX Arietis


Algol Beta Lyrae Planetary transit W Ursae Majoris

Physics Encyclopedia



Hellenica World - Scientific Library

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