In early 1860's, Kirchhoff made three statements about spectral formation which have become known as Kirchhoff's Laws of Spectral Formation. These statements allow one to predict when one should see continuous spectra, emission line spectra, and absorption line spectra.

Kirchhoff's Laws:

• A hot opaque body, such as a hot, dense gas produces a continuous spectrum

• A hot, transparent gas produces an emission line spectrum

• A cool, transparent (dilute) gas in front of a hotter source of continuous emission produces an absorption line spectrum.

This implies that the continuous emission from stars comes from the dense, hot regions of the atmosphere while the absorption lines are formed in cooler layers overlying the opaque regions. This picture is roughly correct, however, we know that it is an oversimplification. In reality, the continuum and the spectral lines are formed over larger regions and the regions over which they are formed are not distinct from one and another.