An Orion variable is a
variable star
A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as e ...
which exhibits
irregular and eruptive variations in its
luminosity and is typically associated with diffuse
nebulae. It is thought that these are young stars which will later become regular, non-variable stars on the
zero-age main sequence. Brightness fluctuations can be as much as several magnitudes.
T Tauri star
T Tauri
T Tauri is a variable star in the constellation Taurus, the prototype of the T Tauri stars. It was discovered in October 1852 by John Russell Hind. T Tauri appears from Earth amongst the Hyades cluster, not far from ε Tauri, but i ...
stars are Orion variables exhibiting characteristic fluorescent violet emission lines from singly
ionize
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
d
iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
(FeII) in their
star spectra, and also emission from
lithium
Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid ...
, a metal that usually is destroyed by the nuclear fusion in the stars.
FU Orionis
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 ...
stars or simply "Fuors", are Orion variables that rise 5–6 magnitudes, then sink up to one magnitude and stay there for many decades. The prototype is
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 ...
, and other specimens are V1057 Cygni and V1515 Cygni.
Variability
Of this diverse class of stars, some Orion variables may exhibit a small amplitude (up to 1
magnitude
Magnitude may refer to:
Mathematics
*Euclidean vector, a quantity defined by both its magnitude and its direction
*Magnitude (mathematics), the relative size of an object
*Norm (mathematics), a term for the size or length of a vector
*Order of ...
) periodic variation, some are characterized by abrupt fadings, and some show spectral characteristics indicating mass downfall upon the star (YY Orionis stars). Many of these characteristics may occur in any one Orion variable.
The term 'Orion Variable' was a handy catch-all term but is now tending to drop out of disuse among the astronomical community, though for historical reasons the GCVS still uses it. Astronomers use more specialised terms which refer to actual physical differences among the 'zoo' of young variable stars, such as 'Classical T Tauri' or 'UX Orionis' stars.
References
*Samus N.N., Durlevich O.V., et al. ''Combined General Catalog of Variable Stars (GCVS4.2, 2004 Ed.)''
*Glasby, J. S. '' The Nebular Variables: International Series of Monographs in Natural Philosophy''
*
External links
GCVS Variability Types
{{Variable star topics
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