What is Spectroscopy?

Spectroscopy is the study of how light interacts with matter, a technique that has become indispensable in modern astronomy. When light from a star, galaxy, or planet passes through a dispersive element—such as a prism or diffraction grating—it spreads into a spectrum of colors, much like a rainbow. Within this spectrum, dark absorption lines or bright emission lines appear at specific wavelengths. These lines are the unique fingerprints of elements and molecules; each atomic or molecular species absorbs or emits light only at certain wavelengths dictated by the quantum mechanical energy levels of its electrons. By analyzing the pattern and intensity of these spectral lines, astronomers can deduce not only the chemical composition of a celestial body but also its temperature, density, pressure, magnetic fields, and radial velocity through the Doppler shift. This powerful technique has been used for over a century and remains the primary way we know what stars are made of across the universe.

The foundation of astronomical spectroscopy was laid in the early 19th century when Joseph von Fraunhofer observed hundreds of dark lines in the Sun’s spectrum. Later, Gustav Kirchhoff and Robert Bunsen linked these lines to specific elements, establishing spectroscopy as a quantitative science. Today, space-based observatories like the Hubble Space Telescope and ground-based instruments like the Very Large Telescope routinely record high-resolution spectra of objects billions of light-years away.

At its core, spectroscopy exploits the fact that each atomic element has a unique set of energy levels. When an electron drops from a higher to a lower energy level, it emits a photon of a specific wavelength. Conversely, an atom can absorb a photon whose energy exactly matches the gap between two levels. This quantum-locked relationship makes spectroscopy an exquisitely precise tool—capable of detecting trace amounts of elements in gas clouds light-years away or identifying isotopic variations in Martian rocks.

Types of Spectroscopy in Astronomy

Astronomers employ several spectroscopic techniques depending on the nature of the celestial object and the information they seek. Each method exploits a different physical process to reveal composition and conditions, and each wavelength regime adds a new dimension to our understanding.

Absorption Spectroscopy

In absorption spectroscopy, astronomers observe the light from a hot, continuous source (like a star) that passes through a cooler, intervening gas. The gas absorbs specific wavelengths, creating dark lines in the spectrum. By matching these lines to laboratory data, scientists can identify the elements present in stellar atmospheres, interstellar clouds, or the outer layers of a planet. For example, the absorption lines of hydrogen (the Balmer series) are used to classify stars into spectral types (O, B, A, F, G, K, M), each corresponding to a distinct temperature and composition. The strengths of these lines also reveal the relative abundances of elements—critical data for models of stellar evolution and nucleosynthesis.

Emission Spectroscopy

Emission spectroscopy focuses on light emitted by hot, ionized gases. When atoms or molecules are excited—typically by high temperatures or radiation—they release photons at characteristic wavelengths, producing bright emission lines. This technique is especially useful for studying nebulae such as the Orion Nebula, where ionized hydrogen glows in the red H-alpha line, or for analyzing the corona of the Sun. Emission lines also reveal the presence of specific ions like doubly ionized oxygen (O III) in planetary nebulae, helping to trace the chemical enrichment of the interstellar medium. In active galactic nuclei, broad emission lines indicate fast-moving gas close to the central supermassive black hole, while narrow lines trace slower gas further out.

Reflectance Spectroscopy

Reflectance spectroscopy measures sunlight or starlight reflected off the surface of a solid body—such as a moon, asteroid, or planet—and decomposes it into a spectrum. Surface minerals absorb and reflect light differently across wavelengths. By comparing the shape of the reflectance spectrum with laboratory spectra of known minerals, scientists can map the mineralogy of planetary surfaces without a physical sample. NASA’s Mars Reconnaissance Orbiter used orbiting spectrometers to identify hydrated clays and sulfate salts on Mars, key evidence for past liquid water. Similarly, the Moon Mineralogy Mapper on India’s Chandrayaan-1 mission discovered hydroxyl and water molecules in sunlit lunar regions.

Infrared and Ultraviolet Spectroscopy

Infrared (IR) spectroscopy is essential for seeing through cosmic dust that obscures visible light. IR spectra reveal vibrational transitions in molecules such as water, carbon dioxide, methane, and organic compounds. The James Webb Space Telescope’s NIRSpec instrument operates in the near-infrared, capturing the emission of cool stars, protoplanetary disks, and the earliest galaxies. Ultraviolet (UV) spectroscopy, on the other hand, probes hot, young stars and the tenuous intergalactic medium. The Hubble Space Telescope’s Cosmic Origins Spectrograph (COS) uses UV lines of hydrogen and heavy elements to trace the structure of the universe’s cosmic web. Each regime offers unique information: IR for molecular fingerprints, UV for atomic transitions at high energies.

Applications of Spectroscopy in Space Exploration

Spectroscopy has transformed our understanding of the cosmos, providing concrete data on the chemical makeup of everything from asteroids to distant quasars. Its applications are as diverse as the objects themselves, extending from our solar system to the edge of the observable universe.

Stellar Composition and Evolution

The composition of stars directly reveals their origins and life cycles. Spectroscopy allows astronomers to measure the abundance of hydrogen, helium, carbon, nitrogen, oxygen, and heavier elements (metallicity) in a star. Older stars tend to have lower metallicity because they formed before the universe was enriched by supernovae. By studying spectral lines in thousands of stars, researchers have reconstructed the chemical evolution of the Milky Way. For example, the Gaia mission combined spectroscopy with astrometry to create the largest, most detailed 3D map of stellar motions and chemistry. Furthermore, Doppler shifts in spectral lines provide stellar radial velocities, enabling the detection of exoplanets via the radial velocity method and the measurement of binary star orbits.

Planetary Science

For planets and moons, spectroscopy identifies surface minerals, atmospheric gases, and even the presence of ice or organic compounds. For instance, the Cassini mission’s Visual and Infrared Mapping Spectrometer (VIMS) revealed the composition of Saturn’s moons, including the hydrocarbon lakes on Titan. On Earth’s Moon, the Moon Mineralogy Mapper discovered hydroxyl molecules trapped in the polar craters. In the atmospheres of gas giants, spectral features of methane, ammonia, and water are routinely observed, helping to refine models of these planets’ deep structure. Upcoming missions like the Europa Clipper will carry an ultraviolet spectrograph to analyze plumes erupting from Jupiter’s moon Europa, searching for organic compounds and salts. The Dragonfly mission to Titan will use a mass spectrometer—a related technique that measures molecular mass rather than light—to directly measure the composition of Titan’s surface and atmosphere.

Exoplanet Atmospheres

One of the most exciting applications is the characterization of exoplanet atmospheres. When an exoplanet transits its host star, some starlight passes through the planet’s upper atmosphere before reaching Earth. Absorption spectroscopy of that filtered light can detect molecules such as water vapor, carbon dioxide, methane, and even potential biosignatures like oxygen or phosphine. The James Webb Space Telescope (JWST) has already provided high-resolution spectra of exoplanets like WASP-39b, confirming the presence of carbon dioxide and sulfur dioxide—a sign of photochemical reactions. These observations are the first steps toward finding signs of life on temperate Earth-like worlds. Emission spectroscopy of exoplanets in the infrared directly measures their day-side temperatures and chemical inventories, revealing weather patterns and possible cloud layers.

Galaxies and the Interstellar Medium

On the largest scales, spectroscopy of galaxies reveals their redshift (i.e., distance), star formation rates, and chemical enrichment history. Emission lines from ionized gas (such as H-alpha and O III) are used to map star-forming regions across galaxies. Absorption lines from interstellar gas along the line of sight to distant quasars probe the distribution of hydrogen (the Lyman-alpha forest) and heavy elements in the intergalactic medium, giving insight into galaxy formation and the rise of metals over cosmic time. Recent surveys like the Dark Energy Spectroscopic Instrument (DESI) are mapping millions of galaxy spectra to understand dark energy and the expansion history of the universe. Spectroscopy of the cosmic microwave background via the Planck satellite has even allowed us to measure the primordial chemical composition of the universe—about 75% hydrogen, 25% helium by mass—with remarkable precision.

Small Bodies: Asteroids, Comets, and Meteorites

Spectroscopy of asteroids helps classify them into compositional types (C‑type, S‑type, M‑type, etc.), which are linked to meteorite classes found on Earth. For example, the Hayabusa2 mission confirmed that asteroid Ryugu is a carbonaceous C‑type body, rich in volatiles and organic compounds. The OSIRIS-REx mission to Bennu used visible and infrared spectroscopy to map its surface mineralogy, including abundant phyllosilicates and carbonates. Cometary spectroscopy reveals the composition of ices—water, carbon dioxide, carbon monoxide, and organic molecules—which are primitive relics from the early solar system. These data are critical for understanding the building blocks of planets and the origin of water and organics on Earth. Ground-based spectroscopy of comets, such as those observed by the Keck Observatory, tracks the outgassing of molecules as they approach the Sun.

Recent Spectroscopic Discoveries

In the last decade, spectroscopy has produced several landmark discoveries that have reshaped planetary science and astrophysics.

  • Water on the Moon: Using reflectance spectroscopy, NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) detected water molecules in sunlit regions of the Moon in 2020, challenging earlier assumptions that water was confined to permanently shadowed poles.
  • Biosignatures in the Atmosphere of a Venus-like World: In 2020, researchers reported the detection of phosphine in Venus’s atmosphere using millimeter-wave spectroscopy, sparking heated debate about potential biological sources. Although later studies questioned the finding, it highlighted the power of spectroscopy to search for anomalous gases.
  • Organic Molecules on Mars: The Sample Analysis at Mars (SAM) instrument on the Curiosity rover used mass spectrometry and gas chromatography (which rely on similar principles) to identify organic compounds like thiophenes and benzene in Martian rocks, indicating a complex carbon chemistry.
  • Heavy Metal Vapors in Exoplanet WASP-76b: High-resolution spectroscopy of the ultra-hot Jupiter WASP-76b detected ionized calcium and iron in its atmosphere, with evidence of iron rain and strong atmospheric winds, providing a glimpse into extreme planetary weather.
  • First Direct Spectrum of an Exoplanet with JWST: In 2022, JWST’s NIRSpec instrument captured a transmission spectrum of the exoplanet WASP-39b, revealing a wealth of information: unambiguous detection of carbon dioxide, water vapor, sodium, potassium, and sulfur dioxide, marking the most detailed exoplanet atmosphere analysis to date.

These examples underscore how spectroscopy continues to deliver surprising results, pushing the boundaries of what we can learn from light.

The Future of Spectroscopy

The next generation of instruments will take spectroscopy even further, especially in the search for life. The James Webb Space Telescope is already revolutionizing exoplanet spectroscopy, but upcoming telescopes like the European Extremely Large Telescope (E-ELT) will have even larger mirrors and advanced spectrographs capable of directly imaging temperate rocky planets and analyzing their atmospheres in reflected light. On the ground, the Legacy Survey of Space and Time (LSST) will produce vast numbers of spectra of transient objects, while high-dispersion spectrographs like ESPRESSO and MAROON-X are pushing radial velocity precision to detect Earth-mass planets around Sun-like stars.

In solar system exploration, the Europa Clipper will carry an ultraviolet spectrograph to analyze plumes erupting from Jupiter’s moon Europa, searching for organic compounds and salts. The Dragonfly mission to Titan will use a mass spectrometer (another form of spectroscopy) to measure the composition of the surface and atmosphere directly. Meanwhile, laboratory spectroscopy continues to build reference databases of mineral spectra and gas-phase molecular lines under planetary conditions, enabling ever more accurate interpretation of astronomical data. The Planetary Society supports citizen science projects that help classify spectral data from missions like Kepler and TESS.

Machine learning and automated spectral classification are also emerging: algorithms can now quickly sort through millions of spectra to identify rare objects like metal-poor stars or exotic supernovae. This synergy between instrumentation, computation, and modeling promises to accelerate discoveries in the coming decades. Additionally, new techniques like spectropolarimetry—which combines spectroscopy with the measurement of polarized light—offer insights into magnetic fields and the geometry of emitting regions around stars and black holes.

Conclusion

Spectroscopy is far more than a technique—it is the bridge between the light that reaches our telescopes and the physical reality of cosmic objects. By decoding the spectral fingerprints imprinted in that light, astronomers have determined the chemical makeup of stars, planets, galaxies, and nebulae, charted the history of the universe's chemical enrichment, and begun to explore the atmospheres of exoplanets for signs of habitability. From the first Fraunhofer lines to the cutting-edge spectra from JWST, spectroscopy remains a cornerstone of astronomical science, continuously unlocking the secrets of what lies beyond our world. As new observatories come online and our analytical methods grow more sophisticated, spectroscopy will continue to illuminate the universe’s composition in ever finer detail, bringing us closer to answering fundamental questions about our cosmic origins.