Astronomy
Using Spectroscopy to Determine the Chemical Composition of Stars
Quick fact
The same technique that reveals the chemical makeup of stars was originally used to discover the first and only known element in the Sun—helium—decades before it was found on Earth.
Why this is interesting
When you look at the night sky, have you ever wondered how astronomers know exactly what distant stars are made of, without ever touching them?
Read the full explanation
Understanding Using Spectroscopy to Determine the Chemical Composition of Stars
Imagine you are standing in front of a prism. When sunlight passes through it, it spreads into a rainbow of colors—a continuous spectrum. But if you look closely at the Sun's spectrum, you'll notice thin dark lines cutting across the colors. These are absorption lines, like a barcode or fingerprint specific to each element. Here's how it works: The inner layers of a star produce a continuous spectrum of light. As that light passes through the cooler outer layers (the stellar atmosphere), atoms in those layers absorb specific wavelengths of light. Each chemical element has a unique set of energy levels for its electrons, so only certain photons (with exactly the right energy) get absorbed. This leaves a pattern of dark lines at those specific wavelengths. By identifying the positions and patterns of these lines, astronomers can determine exactly which elements are present in the star. For instance, a pair of strong dark lines in the yellow part of the spectrum famously led to the discovery of sodium in the Sun.
A deeper explanation
The underlying principle is quantum mechanics: electrons in atoms can only occupy discrete energy levels. When a photon has energy exactly equal to the difference between two energy levels, the electron can absorb it and jump to a higher level, removing that wavelength from the observed spectrum. Because each element has a unique arrangement of energy levels, the set of absorption lines forms a unique signature. This is why spectroscopy is so powerful—it acts as a chemical fingerprint. Astronomers collect starlight using telescopes and feed it into a spectrograph, which disperses the light using a diffraction grating or prism to produce a detailed spectrum. By comparing the observed lines to laboratory measurements, they can identify elements and even measure their abundances by analyzing line strengths. This technique has revealed that most stars are composed primarily of hydrogen and helium, with trace amounts of heavier elements that vary with stellar age and type. It has also enabled the discovery of elements like helium before they were found on Earth, and it allows astronomers to study the chemical evolution of galaxies, the life cycles of stars, and the conditions inside distant exoplanet atmospheres.