Spectroscopy is the process of breaking down light from a source into its component colors to examine the properties of the light source. The detailed pattern of colors that is produced by a light source is called a spectrum. Spectroscopy is like photometry, but in much greater detail. Instead of having relatively large bandpasses, spectroscopy is like photometry with bandpasses just a few nanometers wide, or a few tenths of a nanometer wide, or sometimes even much smaller than that.

A spectrum can be much more complicated than the rainbow of violet (purple), indigo, blue, green, yellow, orange, and red colors with which we are most familiar. When atoms and molecules interact with the light emitted from a source, they can change the spectrum significantly, adding or reducing light in certain colors and patterns. These changes make it possible for astronomers to deduce many of the properties of the light source, and of the intervening material between Earth and the source. Thus, spectroscopy is one of the most important data-analyzing methods astronomers use to learn about the universe.