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Spectroscopy is the investigation and measurement of spectra produced when materials interacts with or emits electromagnetic radiation. Spectra is the plural for spectrum which is a particular wavelength from the electromagnetic spectrum. Common spectra include the different colors of visible light, X-rays, ultraviolet waves, microwaves, and radio waves. Each beam of light is a unique mixture of wavelengths that combine across the spectrum to make the color we see. The light wavelengths are created or absorbed inside atoms, and each wavelength signature matches a specific element. Even white light from the Sun, which seems like an uninterrupted continuum of wavelengths, has gaps in some wavelengths. The gaps correspond to elements present in the Earth’s atmosphere that act as filters for specific wavelengths. These missing wavelengths were famously observed by Joseph von Fraunhofer (1787–1826) in the early 1800s, but it took decades before scientists were able to relate the missing wavelengths to atmospheric filtering. Spectroscopy shows that the Sun is mostly made of hydrogen and helium. Applying this process to light from distant stars, scientists can calculate the abundance of elements in a specific star and visible universe as a whole. Also, this spectroscopic information can be used as an interstellar speedometer.

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The Doppler effect is the same process that changes the pitch of the sound of an approaching car or ambulance from high to low as it passes. When an object emits waves, such as light or sound, while moving toward an observer, the wavelengths get compressed. In sound, this results in a shift to a higher pitch. When an object moves away from an observer, the wavelengths are extended, producing a lower pitched sound. The Doppler effect is used on light emitted from stars and galaxies to determine their speed and direction of travel. Scientists, including Vesto Slipher (1875–1969) and Edwin Hubble (1889–1953), examined galaxies both near and far and found that almost all galaxies outside of our galaxy are moving away from each other, and us. Because the light wavelengths of receding objects are extended, visible light is shifted toward the red end of the spectrum, called a redshift. In addition, Hubble noticed that galaxies that were farther away from Earth also had the greater amount of redshift, and thus, the faster they are traveling away from us. The only way to reconcile this information is to deduce the universe is still expanding. Hubble’s observation forms the basis of big-bang theory.