stellar evolution Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...

, an FU Orionis star (also FU Orionis object, or ''FUor'') is a

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 ...

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 In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

dKe to F-type

supergiant 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 temperature range of supergiant stars spa ...

during 1969-1970. These stars are named after their type-star,

FU Orionis FU Orionis is a variable and binary star system in the constellation of Orion, that in 1937 rose in apparent visual magnitude from 16.5 to 9.6, and has since been around magnitude 9. The name ''FU Orionis'' is a variable star designati ...

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 An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other ...

onto a young, low mass

T Tauri star T Tauri stars (TTS) are a class of variable stars that are less than about ten million years old. This class is named after the prototype, T Tauri, a young star in the Taurus star-forming region. They are found near molecular clouds and ide ...

. Mass accretion rates for these objects are estimated to be around 10⁻⁴

solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es 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, 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 spectra of FU Orionis stars are dominated by absorption features produced in the inner accretion disc. The spectrum of the inner part produce a spectrum of a F-G

, 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 Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from arou ...

. In FU Orionis stars the disk radiation dominates, which can be used to study the inner parts of the disk. The prototypes of this class are:

, V1057 Cygni, V1515 Cygni, and the embedded

protostar A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a low-mass star (i.e. that of the Sun or lower), it lasts about 50 ...

V1647 Orionis, which erupted in January 2004.

References

*Juhan Frank, Andrew King, Derek Raine (2002). Accretion power in astrophysics, Third Edition, Cambridge University Press. .

External links

The Furor over FUOrs
(15 November 2010)

(18 November 2009) * https://web.archive.org/web/20060831060814/http://www.aavso.org/vstar/vsots/0202.shtml {{Variable star topics Star types Stellar evolution Articles containing video clips

See also

References

External links