How Spectroscopy Reveals the Chemical Composition of Stars

The Doppler effect causes the wavelengths of light from moving objects to shift. When a star moves toward Earth, its spectral lines shift toward shorter wavelengths (blue shift); when moving away, they shift toward longer wavelengths (red shift). This allows measurement of stellar velocities. The radial velocity method detects exoplanets by measuring the Doppler shift in a star's spectrum as it wobbles due to the gravitational pull of an orbiting planet. This technique was used to discover the first exoplanets in 1995 and has found over 500 exoplanets to date. The star and planet orbit their common center of mass, causing the star to move in a small circle. By measuring the periodic shifts in the star's spectrum, astronomers can detect the presence of planets even when they cannot be directly imaged.

The Doppler effect causes spectral lines to shift toward the blue end of the spectrum when a star is moving toward Earth and toward the red end when moving away. This is called blue shift and red shift respectively. By measuring these shifts in stellar spectra, astronomers can determine the radial velocity (speed toward or away from Earth) of stars. This technique was first used in the late 1800s by Vögel and Schneider, who measured the Earth's orbital motion around the Sun by detecting the Doppler shift in starlight. Modern astronomers use this technique to detect exoplanets by measuring the wobble of stars caused by orbiting planets.

The Doppler effect describes how light waves change when a light source moves relative to an observer. When a star moves toward Earth, light waves compress, shortening the wavelength and shifting the spectrum toward blue (blueshift). When a star moves away, waves stretch, lengthening the wavelength and shifting toward red (redshift). Radial velocity is the component of stellar motion along the line of sight from Earth. When a star approaches Earth, radial velocity is negative; when it recedes, radial velocity is positive. These concepts form the foundation for detecting exoplanets through stellar motion.

The Doppler effect describes how wave frequencies change based on relative motion between source and observer—for sound, this causes a car horn to sound higher-pitched when approaching and lower when receding. In astronomy, this manifests as redshift (light shifted toward longer wavelengths, indicating motion away) and blueshift (light shifted toward shorter wavelengths, indicating motion toward). The radial velocity method applies this principle to detect exoplanets: as a planet orbits its star, both bodies orbit their shared center of gravity, causing the star to wobble periodically. By measuring periodic redshift and blueshift in a star's spectrum, astronomers can infer the existence, mass, and orbital distance of orbiting planets.

The Doppler effect causes spectral line shifts when light sources move relative to observers. Motion away causes redshift (longer wavelengths), while motion toward causes blueshift (shorter wavelengths). This principle enables radial velocity measurements for detecting exoplanets through stellar wobble. The shift depends on velocity component along the line of sight, with the relationship Δλ/λ = v/c. For planetary atmospheres, Doppler shifts reveal wind patterns and atmospheric circulation. The technique was recognized with the 2019 Nobel Prize in Physics for exoplanet detection.

Inside stars, nuclear fusion creates heavier elements from lighter ones. The first stars created the first heavy elements, which were then dispersed into space by supernova explosions. Subsequent generations of stars formed from this enriched material, creating progressively heavier elements. This process, called stellar nucleosynthesis, is responsible for the chemical diversity of the universe.

Stars are plasma spheres where chemistry does not occur, but nuclear reactions (alchemy) do. Stars synthesize heavier elements from lighter ones through nuclear fusion. On final evolutionary stages, stars eject their outer layers, and if this occurs with a supernova explosion, additional element synthesis happens. Elements synthesized outside the core and deposited in the envelope are also important for the chemical evolution of the universe.

Iron cores cannot generate energy through fusion, causing catastrophic collapse and supernova explosions. Massive stars leave neutron star remnants (10 km diameter, solar mass). Elements heavier than iron form through neutron capture: the s-process in stars produces elements up to bismuth, while the r-process in supernovae or neutron star mergers produces very neutron-rich isotopes. Lithium, beryllium, and boron are primarily produced by cosmic ray spallation - high-energy cosmic rays breaking heavier nuclei in interstellar space. This explains their low cosmic abundances. The Universe's chemical evolution proceeds through stages: primordial nucleosynthesis (hydrogen and helium), stellar nucleosynthesis (all heavier elements), and distribution through supernovae. This cycle creates the chemical diversity observed in stars, planets, and life.

Chemical elements are forged in the interiors of massive stars through nuclear fusion. In the early universe, stars consisted mainly of hydrogen and helium. These massive stars underwent fusion, converting lighter elements into heavier ones like carbon, oxygen, and nitrogen. When stars exploded as supernovae, they scattered these heavy elements into the interstellar medium, enriching molecular clouds for future star formation. Astronomers can calculate element concentrations in stars to understand cosmic chemical evolution and track how elements are distributed throughout the universe over time.

After the Big Bang, stellar nucleosynthesis created heavier elements. In stellar cores, hydrogen fuses to helium, then helium fuses to carbon, continuing through successive stages to iron in massive stars. Elements heavier than iron form through neutron capture processes (r-process in supernovae, s-process in asymptotic giant branch stars). These elements are dispersed by stellar winds and supernova explosions, seeding interstellar clouds with materials necessary for new star and planet formation, ultimately enabling the complex chemistry essential for life.

NASA detects water and carbon dioxide in exoplanets using transmission spectroscopy, a technique that analyzes the light passing through an exoplanet's atmosphere during a transit; when starlight passes through the atmosphere, molecules absorb specific wavelengths of light, creating a unique spectral signature that reveals the planet's atmospheric composition.

Transmission spectroscopy is a technique used to study exoplanet atmospheres. The process involves measuring the brightness of a star, then measuring the brightness when the exoplanet passes in front of the star, and subtracting the two measurements. The difference reveals the atmosphere's composition. The James Webb Space Telescope used this technique to observe WASP-17b for 10 hours, collecting 1,275 measurements of its atmosphere to identify the chemical elements present.

Transmission spectroscopy is a method used to study exoplanet atmospheres. When an exoplanet transits (passes between its host star and Earth), it blocks some starlight but not all wavelengths equally. Different atmospheric chemicals absorb different wavelengths of light, similar to how Earth's atmosphere blocks ultraviolet light from the Sun. By analyzing which wavelengths are filtered, scientists can determine what chemicals exist in an exoplanet's atmosphere and learn about its composition.

Transmission spectroscopy is the primary method scientists use to analyze exoplanet atmospheres from Earth. When a planet passes between Earth and its star (transit), some starlight passes through the planet's atmosphere. Different gases absorb specific wavelengths of light, creating unique absorption patterns. By measuring which colors get absorbed, scientists can identify which gases are present in the planet's atmosphere, determine atmospheric density, and measure temperature. This technique allows astronomers to study distant planets despite their vast distances.

The James Webb Space Telescope analyzes exoplanet atmospheres using transit spectroscopy. When a planet passes in front of its star (transit), starlight passes through the planet's atmosphere. Atoms in the atmosphere absorb specific wavelengths of light, causing photons of certain energies to be missing from the received light. These missing wavelengths appear as dark lines (absorption lines) in the star's spectrum, creating a unique spectral signature that reveals the chemical composition of the distant atmosphere.