### spectral index

**(α)**
(power by which an object's flux depends on frequency)

The **spectral index** of a source of electromagnetic radiation is a scalar measure
describing its spectrum, assuming the spectrum forms a power law.
It is specifically the exponent of the frequency
that produces a term proportional to the radiant flux at the
same frequency, i.e.,

S ∝ ν^{α}

- S - radiant flux.
- ν - frequency.
- α - spectral index.

Confusingly, sometimes the term is used to mean the negative of
this exponent.
Also, sometimes *spectral index* refers to an exponent of the
wavelength rather than of the frequency.

In cases where the radiant flux by frequency is not a power law,
it can be said that the spectral index varies by frequency.
The concept seems less useful in this case.

(*EMR,measure*)
http://en.wikipedia.org/wiki/Spectral_index

**Referenced by:**

CRATES

Lambda-CDM model (ΛCDM)

USS Sources

young stellar object (YSO)

index