More than anything else, breaking down the light from celestial objects into its constituent colors has helped us understand the universe. A spectrum can tell astronomers what an object is made of, how hot it is, how fast it is moving, and a host of other important attributes. Spectroscopy has revealed the great abundance of hydrogen and helium in the universe—providing observational support for the big-bang theory—and showed the relative amounts of the other elements since cooked up in stars.

       Just as important, spectroscopy revealed the expansion of the universe. When an object moves away from us, the lines in its spectrum get displaced toward longer wavelengths, with the amount of this so-called redshift proportional to the object’s velocity. Edwin Hubble first showed that the spectrum of almost every galaxy is shifted to the red, and that the farther away the galaxy, the greater the redshift. From these observations, cosmologists correctly deduced that the universe is expanding.