Question

Using Kirchoff’s Laws explain the formation of stellar dark line spectra that we see.

Answer 1

Kirchoff's Laws

1. First Law: A hot solid, liquid, or dense gas emits radiation at all wavelengths ("a continuous spectrum of radiation"). For example, a perfect blackbody does this. If the light were passed through a prism, you would see the whole rainbow of colors in a continuous band.
2. Second Law: A thin hot gas in front of a cooler background emits radiation at a discrete set of isolated wavelengths. These discrete, isolated wavelengths are called the "emission lines" of the spectrum, because if you were to pass the radiation through a prism, you would see isolated lines of different colors. The whole spectrum is called an "emission-line" spectrum. The wavelengths of the emission lines are unique to the type of neutral atom or ionized atom that is producing the emission lines.
3. Third Law: A thin cool gas in front of a hotter solid, liquid, or dense-gas background removes the radiation from the background source at special wave lengths. If the resulting radiation were passed through a prism, there would be dark lines superimposed on the continuous band of colors due to the background. These dark lines are called "absorption lines." The wavelengths of the absorption lines are unique to the type of neutral atom or ionized atom that is producing the emission line                   If a certain type of gas produces absorption lines at certain wavelengths when it is in front of a hot background, then when that same type of gas is seen in front of a cooler background, it produces emission lines at the exact same wavelengths.
4. EXPLANATION
5. Why do the spectra of stars include absorption lines? The reason is that stars are very hot in their cores, but get colder towards their surface. The radiation that we see from a star passes through the outer cooler gas at the star's surface. Since this outer layer is relatively thin, and cooler than the layers underneath, it produces an absorption line spectrum.
6. Why do the spectra of stars include emission lines? Many stars are surrounded by a very hot, thin gas (plasma) that is significantly hotter than the star's surface. Kirchoff's second law in the list above then implies that this hot tenuous gas can give off emission lines.