When we turn on a light in a dark room, it illuminates our surroundings, allowing us to better understand our environment. Astronomers use light in a similar way, but light from astronomical objects not only illuminates the cosmos, it also helps us better understand the very nature of the objects we are looking at.

Spectroscopy, the study of light and its component parts, had its beginnings in the 17th century when Isaac Newton passed a beam of sunlight shining through a pinhole in a covered window through a prism. The prism separated the sunbeam into a continuous series of colors that Newton called a “spectrum.”

Rainbows are the best-known type of spectrum. They form when sunlight bends (refracts) and then bounces (reflects) inside a raindrop. Our Sun emits white light which holds all colors of the rainbow in roughly equal amounts. When sunlight encounters a raindrop, the raindrop acts like the prism in Newton’s experiment. It separates sunlight into its different wavelengths, producing the familiar arc of color we call a rainbow.

But spectroscopy produces more than a simple rainbow arc of color. It can reveal the molecules around stars or in the atmospheres of distant planets. It can tell us their temperatures, densities, and in the case of gas, their pressures. Spectroscopy can also reveal the distance to an object along with the direction and speed it is traveling.

All normal matter in the universe either emits, absorbs, reflects, or transmits light. That light, called electromagnetic radiation, includes the rainbow of visible light our eyes can see, the ultraviolet radiation that streams from the Sun, the infrared or heat radiation we feel when we open a hot oven’s door, X-rays, radio waves, and all of the wavelengths in between or beyond.

Because the range of wavelengths available for study is so broad and each one has different requirements to capture, no one telescope can study them all. That means telescopes specialize in a wavelength range, like the James Webb Space Telescope that studies infrared light, or series of ranges like Hubble.

Hubble’s design and instruments allow it to capture ultraviolet, visible, and a small part of the infrared spectrum called near-infrared. As light from a distant cosmic object enters Hubble, its mirrors reflect and focus that light onto Hubble’s instruments. Those instruments include dedicated spectrographs that separate light into its different wavelengths.

When light from a cosmic object enters one of Hubble’s spectrographs, it passes through a tiny slit in a metal plate that isolates it from the light of other objects. The light then bounces off a special metal grating, called a diffraction grating, that splits the light into its component wavelengths, focusing them onto a detector that records them as a spectrum.

Hubble’s current instruments include two main spectrographs — the Space Telescope Imaging Spectrograph (STIS) that captures wavelengths from ultraviolet through visible and into near-infrared, making it an “all purpose” instrument, and the Cosmic Origins Spectrograph (COS) which measures faint ultraviolet light from faraway sources like remote galaxies— and three cameras — Advanced Camera for Surveys (ACS), Wide Field Camera 3 (WFC3), and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). Each of these cameras also have spectrographic capability that allows astronomers to capture an image along with detailed data about the object’s light.

Astronomers study three basic types of spectra: continuous, absorption, and emission.

A continuous spectrum holds all wavelengths of light across a certain wavelength range. As its name implies, it is uninterrupted with no gaps or intensely bright bands. The hot, compact point of light emitted by stars is nearly a continuous spectrum.

We see an absorption spectrum when light from an object like a star passes through another medium, like an interstellar cloud or nebula. As the light interacts the nebula, the nebula may absorb certain wavelengths of light while others pass through. The absorbed wavelengths depend on the elements and compounds that make up the nebula. An absorption spectrum holds dark lines or breaks that correspond to the absorbed wavelengths of light.

An emission spectrum is the result of excited atoms and molecules that emit light. As starlight passes through a nebula, it can heat up the cloud, exciting gas molecules and elements causing them to glow. The resulting emission spectrum holds a series of bright colored lines that correspond to wavelengths of light emitted by the excited atoms and molecules.

Astronomers look at spectra in the form of a graph that generally holds wavelengths along the x-axis and light intensity (luminous flux) along the y-axis. A spectrum's curve, called a continuum, represents the continuous emission of electromagnetic radiation at all wavelengths. A bright line (emission) or dark line (absorption) in the spectra correlates to peaks and valleys on the curve. They reveal the wavelengths where specific atoms emit or absorb light.

Spectra allow astronomers to easily see the intensity of light at precise wavelengths; providing them with important details about the object’s chemical makeup, temperature, density, motion, or physical environment. They offer a detailed way to compare the spectra of any given object over time, allowing astronomers to track the evolution of an object and its environment. Such detail also allows astronomers to compare objects so they can better determine differences in environment, age, motion, and physical characteristics, making each spectrum a fingerprint of the object it represents.