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Spectroscopy 101 – Introduction

What is spectroscopy, anyway?

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Astronomers have figured out a lot of very, very specific details about the universe. 

We know, for example, that 91.2% of the atoms in the Sun are hydrogen. We know there is water vapor in the atmosphere of WASP-39 b, a distant planet 700 light-years away. We know that the gas at the center of galaxy M82 is 80,000,000 degrees Celsius (150 million degrees Fahrenheit). And that the Whirlpool Galaxy is moving away from us at a rate of 460 kilometers (285 miles) every second.

Astronomy articles are filled with exact compositions, temperatures, and speeds of things that are too far to visit, impractical to probe, and in some cases impossible to observe directly. How is it possible to get such detailed measurements like these?

The answer is spectroscopy. 

galaxy image
Image of Galaxy M82 from the Hubble Space Telescope
Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).
exoplanet illustration
Illustration of exoplanet WASP-39b
Credit: NASA, ESA, and G. Bacon (STScI).
galaxy image
Hubble image of the Whirlpool Galaxy and its companion galaxy NGC 5195
Credit: NASA, ESA, S. Beckwith (STScI), and the Hubble Heritage Team (STScI/AURA).
  • Spectroscopy is a scientific method of studying objects and materials based on detailed patterns of colors (wavelengths).
  • Spectroscopy is used to figure out what things are made of, how hot they are, how dense they are, and how fast they are moving in space.
  • Spectroscopy works because light and matter interact with each other in very specific and predictable ways.
  • Different materials give off and interact with different colors of light in different ways, depending on properties like temperature, composition, and motion.

What is Spectroscopy?

A comparison of the spectrum of the Sun (top half of the graphic) to the spectrum of a fluorescent light bulb (bottom half of the graphic). The two spectra are distinctly different: The spectrum of sunlight is shown as a continuous curve and rainbow. The spectrum of the light bulb consists of a set of discrete sections, shown as peaks on the graph and bands of color in the picture. Select View Description for more details.
Light can be spread out into a rainbow of colors known as a spectrum. A spectrum can be displayed as a picture or on a graph. Different sources of light have different spectra because they have different properties, such as composition and temperature. Get the spectrum comparison infographic.
Credit: NASA, ESA, and L. Hustak (STScI).

While it might sound a bit like an unpleasant medical procedure, at its most basic, spectroscopy is simply a scientific method of studying objects and materials based on color. More specifically, spectroscopy involves analyzing spectra: the detailed patterns of colors (wavelengths) that materials emit, absorb, transmit, or reflect.

Along with imaging (i.e., photography), spectroscopy is one of the most common and useful techniques in astronomy. While images provide information about the size, shape, and structure of matter in space, spectra provide key details such as temperature, composition, and motion.

Spectroscopy is also used in a wide variety of fields outside astronomy, including materials science, Earth science, medicine, forensics, national security, and food safety. 

If you are not already familiar with spectroscopy, it is probably because spectra (which are commonly displayed in graph form) don’t typically have a lot of aesthetic appeal. But while they don’t seem to get much press, spectra are in fact humbly hanging out behind the scenes of some of the most intriguing headlines in astronomy. 

The graphic is divided into three panels under the text “What Can Spectroscopy Tell Us?” The first panel, labeled “Planet,” shows a picture of Mars with two labels: “composition of atmosphere,” pointing to the Martian atmosphere, and “minerals on a planet’s surface,” pointing to the surface of Mars. The second panel, labeled “Nebula,” shows a multicolored, hourglass-shaped nebula inside another multicolored, hourglass-shaped nebula. The panel displays two labels, both of which point to the outer nebula: “velocity of gas” and “density of gas.” The third panel, labeled “Star,” shows the Sun, a yellow-orange orb with surface texture. Two labels point to the star: “temperature” and “speed of rotation.” For more details, select View Description.
Almost everything we know about the make-up, temperature, and motion of planets, stars, and galaxies comes from spectroscopy: measuring the specific colors of light that they emit, absorb, transmit, and reflect. Get the full spectroscopy infographic.
Credits: Hubble image of Mars: NASA, ESA, the Hubble Heritage Team (STScI/AURA), J. Bell (ASU), and M. Wolff (Space Science Institute). Hubble image of the Southern Crab Nebula: NASA, ESA, and STScI. Solar Orbiter image of the Sun: Solar Orbiter/EUI Team/ ESA & NASA; CSL, IAS, MPS, PMOD/WRC, ROB, UCL/MSSL​​​​. Composition: NASA, ESA, and L. Hustak (STScI).

Spectroscopy allows us to identify gases in planetary atmospheres and minerals on planetary surfaces; figure out what stars are made of and how fast they are rotating; detect and characterize planets orbiting distant stars; measure the temperature and speed of gases in the center of an active galaxy; infer the presence of black holes and dark matter; unravel interactions between colliding galaxies; and help calculate the expansion rate and age of the universe.