An Etymological Dictionary of Astronomy and Astrophysics

1. The → exponent of the → frequency on which depends
   the intensity of the → continuum emission, that is: F_ν∝ ν^α. The exponent (α) typically takes positive values from 0 to 2 for → thermal emission,
   while → non-thermal emission, such as → synchrotron radiation, leads to negative values of the spectral index ranging from about -0.5 to -1.5.
   

2. The ratio α_IR = dlog(λF_λ)/dlogλ, where F represents the flux and λ the wavelength, in the range 2.2 μm ≤ λ ≤ 25 μm, particularly used in the classification of → protostars (→ Class I, → Class II, and → Class III).

See also: → spectral; → index.