Spectroscopy means spreading light into its colors, a spectrum, and studying it in detail. It is one of the most powerful tools in astronomy: from a point of light, it can reveal what a star is made of, how hot it is, how fast it moves and much more.
Spectral lines: chemical fingerprints
Every element absorbs and emits light at specific wavelengths. In the early 1800s, Joseph von Fraunhofer noticed hundreds of dark lines in the Sun’s spectrum. Around 1860, Gustav Kirchhoff and Robert Bunsen showed that these lines match elements heated in the laboratory, such as sodium and iron.
- Absorption lines: dark lines where cooler gas in a star’s outer layers absorbs light from the hotter interior.
- Emission lines: bright lines from glowing gas, as in nebulae, see nebulae.
How we know what the Sun is made of
Spectroscopy revealed the Sun’s ingredients. Helium was first detected in the Sun’s spectrum during an 1868 eclipse, decades before it was found on Earth, hence its name from the Greek “helios”. In 1925, Cecilia Payne showed that stars are made overwhelmingly of hydrogen and helium, contrary to the belief that they had Earth-like compositions. Today we know the Sun is about 73 percent hydrogen and 25 percent helium by mass, see the Sun’s role in the solar system.
What else a spectrum reveals
- Temperature: which lines appear depends on temperature, the basis of the O, B, A, F, G, K, M classification, see types of stars.
- Motion: the Doppler effect shifts lines toward blue for approaching objects and red for receding ones. This reveals binary stars, orbiting planets and the expansion of the universe, see binary stars and exoplanets.
- Rotation: spinning stars have broadened lines.
- Magnetic fields: lines split (the Zeeman effect).
- Chemical history: the amount of heavy elements tells astronomers whether a star is old (few metals) or young, and traces how galaxies built up their elements over time.
- Size class: line widths distinguish giants from dwarfs.
Instruments
Astronomers use spectrographs with diffraction gratings attached to telescopes. Modern surveys record the spectra of millions of stars and galaxies at once using fibers or multi-slit masks.
Beyond stars
The same technique identifies molecules in planetary atmospheres and measures the redshift of distant galaxies, see planetary atmospheres. Amateurs can try it too with inexpensive grating filters that fit on an eyepiece.
