FU Orionis is a variable and

binary star A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...

system in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Orion Orion may refer to: Common meanings * Orion (constellation), named after the mythical hunter * Orion (mythology), a hunter in Greek mythology Arts and media Fictional entities Characters and species * Orion (character), a DC Comics c ...

, that in 1937 rose in apparent visual magnitude from 16.5 to 9.6, and has since been around magnitude 9.AAVSO: FU Orionis
/ref> The name ''FU Orionis'' is a

variable star designation In astronomy, a variable-star designation is a unique identifier given to variable stars. It extends the Bayer designation format, with an identifying label (as described below) preceding the Latin genitive of the name of the constellation in whic ...

in the Argelander system, which are assigned sequentially as new variables are discovered. FU Orionis is about 1,360 light years distant and is associated with the molecular cloud Barnard 35. For a long time this variable was considered unique, but in 1970 a similar star,

V1057 Cygni V1057 Cygni is a suspected binary star system in the northern constellation of Cygnus. It is a variable star of the FU Orionis-type, and was the second FU Orionis-type variable to be discovered. The system is located at a distance o ...

, was discovered, and a number of additional examples have been discovered since then. These stars constitute the FU Orionis class of variable stars, GCVS type ''FU'', often nicknamed ''FUors''. These stars are

pre–main sequence star A pre-main-sequence star (also known as a PMS star and PMS object) is a star in the stage when it has not yet reached the main sequence. Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope o ...

s which display an extreme change in magnitude and spectral type.

Stellar system

FU Orionis consists of two components, both surrounded by a

circumstellar disk A Circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the res ...

. Both disks were resolved with

ALMA Alma or ALMA may refer to: Arts and entertainment * ''Alma'' (film), a 2009 Spanish short animated film * ''Alma'', an upcoming film by Sally Potter * ''Alma'' (Oswald de Andrade novel), 1922 * ''Alma'' (Le Clézio novel), 2017 * ''Alma'' ( ...

. The primary is surrounded by a dust disk with a radius of 11

astronomical unit The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...

s and the secondary disk has a similar inclination and size. The disks are separated by about 250 au. The ¹² CO emission show a complex kinematic environment and signatures of disk rotation, which are asymmetric. The asymmetry of the disk rotation is explained with interactions of the disks during a

stellar flyby Stellar flyby refers to the close passage of two or more stars, which remain unbound after their passage Close encounters with the Sun The Sun resides in a region of relatively low stellar density in the Milky Way. Thus, close stellar flybys ar ...

. The primary, called FU Orionis north has a mass of 0.6 and accretes

3.8 \times 10^

per year. The primary has an uncertain spectral type and luminosity class. FU Orionis stars do not show strong emission lines during the outburst and have spectral features that resemble F- or G-type

supergiants Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram, with absolute visual magnitudes between about −3 and −8. The temperatures of supergiant stars range ...

during the maximum. The outer parts of FU Orionis stars produce a K-M supergiant spectrum, which can be observed in the

near-infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of ...

. The secondary, called FU Orionis south could be the more massive component in the system with 1.2 and a spectral type of about K5.

Nebula

FU Orionis is associated with the

molecular cloud A molecular cloud—sometimes called a stellar nursery if star formation is occurring within—is a type of interstellar cloud of which the density and size permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...

Barnard 35 (part of the Lambda Orionis Ring) and close to the star an arc-shaped nebula is visible. Other FU Orionis stars are associated with an arc-shaped

reflection nebula In astronomy, reflection nebulae are interstellar cloud, clouds of Cosmic dust, interstellar dust which might reflect the light of a nearby star or stars. The energy from the nearby stars is insufficient to Ionization, ionize the gas of the nebu ...

that becomes visible as the star brightens.

Hypothetical planet

A 2023 study proposed that FU Orionis north might be accreting matter from an evaporating planet about 6 times the mass of Jupiter. Simulations predict an extremely large radius of or around at the beginning of the extreme evaporation event.

References

External links

AAVSO Variable Star of the Month. FU Ori: February 2002

FU Orionis
at NightSkyInfo.com {{DEFAULTSORT:FU Orionis FU Orionis stars G-type supergiants Orion (constellation) Durchmusterung objects Orionis, FU Binary stars