Determining the chemical makeup of the cosmos is challenging due to distance and scale, making it impossible to collect physical samples from distant stars and galaxies. Scientists must rely on light, the only messenger that travels across space. The process hinges on the principle that matter interacts with electromagnetic radiation in a specific and measurable way. By capturing and analyzing this light, researchers decode the elemental composition of celestial objects, from exoplanet atmospheres to the remnants of exploded stars.
How Atoms Interact with Light
The ability to identify elements remotely stems from the quantum mechanical structure of the atom. Electrons are restricted to specific, discrete energy levels or shells, and they cannot exist between these levels. An atom will only absorb a photon if the photon’s energy precisely matches the difference between two electron energy levels. This absorption causes the electron to jump from a lower level to a higher, temporary excited state.
Conversely, an electron in an excited state will spontaneously fall back down to a lower energy level, releasing the excess energy as a new photon. The energy of this emitted photon is exactly defined by the difference between the initial and final energy levels. Because every element, such as hydrogen or iron, possesses a unique configuration of protons and electrons, its set of energy level differences is entirely unique. This results in each element having a distinct set of wavelengths it can absorb or emit, which forms its spectral signature.
Reading Elemental Fingerprints Through Spectroscopy
The spectral signature of an element is observed using spectroscopy, where a spectrograph splits light into its component wavelengths. When light from a hot, dense source, like a star’s interior, passes through a cooler gas cloud, the atoms absorb specific wavelengths corresponding to their energy transitions. This absorption creates an absorption spectrum, which appears as a continuous rainbow of light crossed by a pattern of dark lines.
If the gas cloud itself is hot and glowing without a continuous background source, the excited atoms emit their characteristic photons, producing an emission spectrum. This spectrum is characterized by a series of bright, isolated lines against a dark background. By comparing the precise pattern and position of these dark or bright lines against laboratory-created reference spectra for known elements, astronomers can definitively identify the chemical composition of distant stars, nebulae, and galactic gas clouds. For example, the presence of a specific triplet of lines in the yellow-orange region of a star’s spectrum immediately identifies the element sodium.
Mapping Element Abundance Across the Universe
Once an element is identified by its spectral lines, the next step is to determine its quantity, or abundance, in the celestial object. The intensity—the darkness of an absorption line or the brightness of an emission line—correlates with the number of atoms present. A stronger, deeper line indicates a greater abundance of that element. However, simply measuring line intensity is not enough, as the strength of a line is highly dependent on the temperature and density of the gas.
If the gas is too hot, atoms may become ionized, losing electrons and changing their spectral signature, which complicates the abundance calculation. Researchers must employ sophisticated models to account for the physical conditions of the star or nebula, using multiple spectral lines from different ionization states of the same element to accurately calculate its total concentration. The analysis is not limited to visible light; telescopes and satellites utilize the full electromagnetic spectrum to probe different environments. X-ray spectroscopy, for instance, identifies highly ionized heavy elements like iron and nickel in million-degree plasma found in supernova remnants and black hole accretion disks.
Verifying Composition Closer to Home
While spectroscopy provides the vast majority of knowledge about the universe’s composition, physical samples collected within our solar system offer a direct way to verify remote findings. Analysis of meteorites, particularly carbonaceous chondrites, gives scientists a pristine baseline of the early solar nebula’s composition, the starting material for the Sun and planets. These samples are analyzed using laboratory instruments for their precise elemental and isotopic ratios.
Space missions have provided “ground truth” samples. For instance, the Apollo missions returned hundreds of kilograms of lunar rocks, and the Genesis mission collected solar wind particles. Laboratory analysis of these collected solar wind ions, including elements like oxygen, nitrogen, and magnesium, provides a high-precision chemical check against the spectroscopic analysis of the Sun’s outer layers. These samples calibrate and confirm the models and remote measurements derived from spectroscopic techniques.