Spectral sources

Different types of light sources have different spectral characteristics. There are three important simple types. They are:

1. glowing ("incandescent") solids (or high density liquids or gases), which emit "continuous" spectra (all colors);

2. hot, electrically excited gases which give ``bright line'' or "emission" spectra (definite colors only, no light at all except at those colors);

3. a continuous spectrum, observed through a cold gas, which is unchanged except that certain definite colors are missing; that is some radiation is absorbed by the cold gas, and the spectrum is called an "absorption" spectrum.

Thus observing the spectrum of a light source can tell us about the physical state of the source as well as its constitution. We get only light from stars. So it is vital to exploit it as much as possible.