Astronomy
Stellar Spectra
Quick fact
By analyzing a star's spectrum, astronomers can determine it contains hydrogen, helium, and other elements – the same technique works for distant galaxies and exoplanets.
Why this is interesting
When you look at a star, you see its color. But hidden in that light is a barcode that tells you everything about the star—its temperature, composition, and even how fast it's moving.
Read the full explanation
Understanding Stellar Spectra
Starlight is a mixture of all colors of the rainbow. When this light passes through a prism or diffraction grating, it spreads out into a spectrum. You might expect a smooth rainbow, but instead you see dark lines crossing it at specific positions. These are absorption lines, created when atoms in the star's cooler outer layers absorb certain wavelengths of light. Each chemical element has a unique set of lines, like a fingerprint. For example, hydrogen always produces a series of lines at specific positions. By matching the pattern of lines, we can identify which elements are present in a star. The overall shape of the spectrum—how bright it is at different colors—also tells us the star's surface temperature. A hot star like Sirius appears blue and emits most light at shorter wavelengths, while a cool star like Betelgeuse appears red and peaks at longer wavelengths. This continuous background is called blackbody radiation, and its peak wavelength is inversely related to temperature.
A deeper explanation
The mechanism behind stellar spectra involves three key physical processes: thermal emission, atomic absorption, and Doppler shifting. The star's hot interior produces a continuous spectrum via blackbody radiation—the intensity and peak wavelength depend on temperature. This continuous light then passes through the star's cooler, low-density atmosphere (the photosphere). Here, atoms and ions absorb photons at very specific energies that correspond to transitions of electrons between energy levels. For example, a hydrogen atom's electron can jump from the n=2 to n=4 level only by absorbing a photon of a precise wavelength (486.1 nm, the H-beta line). This removes those photons from the continuous spectrum, creating dark absorption lines. The pattern of lines is determined by the atomic structure of each element and by the star's temperature and pressure, which affect the ionization and excitation states. Additionally, if the star is moving toward or away from Earth, the entire pattern of lines shifts due to the Doppler effect: moving toward compresses wavelengths (blueshift), moving away stretches them (redshift). This shift reveals the star's radial velocity and is crucial for detecting exoplanets and measuring cosmic expansion. Stellar spectra are thus decoded to assign spectral types (O, B, A, F, G, K, M) based on temperature and line strength, forming the Harvard classification system. This classification directly links to a star's mass, luminosity, and evolutionary stage, making stellar spectra a cornerstone of modern astrophysics.