Orion variable

T Tauri stars are Orion variables exhibiting characteristic fluorescent violet emission lines from singly ionized iron (Fe II) in their star spectra, and also emission from lithium, a metal that usually is destroyed by the nuclear fusion in the stars.

FU Orionis stars or simply "Fuors", are Orion variables that rise 5–6 magnitudes, then sink up to one magnitude and stay there for many decades.

Of this diverse class of stars, some Orion variables may exhibit a small amplitude (up to 1 magnitude) periodic variation, some are characterized by abrupt fadings, and some show spectral characteristics indicating mass downfall upon the star (YY Orionis stars).

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.