Kirchoff's Laws

The three laws below constitute Kirchoff's Laws:
 * 1) A hot solid object produces light with a continuous spectrum
 * 2) A hot tenuous gas produces light at discrete wavelengths which depend on the energy levels of the atoms in the gas
 * 3) A hot solid object surrounded by a cool tenuous gas will produce an almost continuous spectrum which has gaps at discrete wavelengths depending on the energy levels of the atoms in the gas

Using Kirchoff's Laws to Explain Line Broadening
Kirchoff's Laws can be used to explain line broadening

Natural Broadening
We could see naturally broadened absorption lines when a cool gas cloud is in front of a continuum source such as a star, which is the case of kirchoff's 3rd law.

Doppler Broadening
We could see Doppler broadened emission lines when a hot gas is at low pressure, in the case of Kirchoff's 2nd law. The randomly moving hot gas particles will produce the Doppler Broadening

Collisional Broadening
We can see collisional broadening with a hot gas at high pressure, the case of Kirchoff's 1st law. This hot gas will emit a continuum because collisional broadening can broaden lines so much that they blend together into a continuum (although hot gas at high pressure also produces radiation through mechanisms other than electron de-excitation)