Spectra and what they reveal
Spreading light into a spectrum reveals what celestial objects are made of, how hot they are, and how they move.
Spectroscopy
A spectrograph spreads light into its component wavelengths, like a prism making a rainbow. Spectroscopy is one of astronomy's most powerful tools, revealing composition, temperature, density, motion, and magnetic fields of objects we can never visit.
Kirchhoff's laws
Gustav Kirchhoff described three types of spectra. A hot, dense object produces a continuous spectrum. A hot, thin gas produces an emission spectrum of bright lines. A cooler, thin gas in front of a hotter source produces an absorption spectrum: a continuous spectrum with dark lines. Stars show absorption spectra.
Atoms and spectral lines
Electrons in atoms occupy specific energy levels. When an electron drops to a lower level, the atom emits a photon of a specific energy and wavelength; when it absorbs a photon of exactly the right energy, the electron jumps up. Each element has a unique pattern of lines, like a fingerprint.
Discovering helium
In 1868, astronomers observing the Sun's spectrum during an eclipse found a bright yellow line that matched no known element. It was named helium, from the Greek helios, Sun. Helium was found on Earth only in 1895. Spectroscopy thus discovered an element in space before it was found at home.
Stellar composition
In 1925, Cecilia Payne-Gaposchkin showed from stellar spectra that stars are made mostly of hydrogen and helium, overturning the belief that they had Earth-like compositions. Today we know the Sun is about 73% hydrogen and 25% helium by mass, with about 2% heavier elements.
The Doppler effect
When a light source moves toward us, its wavelengths are compressed (blueshift); when it moves away, they are stretched (redshift). The amount of shift reveals the speed along our line of sight. Doppler measurements reveal binary stars, the rotation of galaxies, and planets around other stars.
Finding exoplanets
A planet's gravity makes its star wobble slightly, producing tiny periodic Doppler shifts. In 1995, Michel Mayor and Didier Queloz used this radial velocity method to discover 51 Pegasi b, the first planet found orbiting a Sun-like star, earning a share of the 2019 Nobel Prize in Physics.
Worked example
A spectral line normally at 656.3 nm appears at 657.6 nm in a star's spectrum. The shift is 1.3 nm, a redshift, so the star is moving away. Its speed is \(v = c \times \frac{\Delta\lambda}{\lambda} = 300{,}000 \times \frac{1.3}{656.3} \approx 594\) km/s.
Key takeaways
Spectroscopy reveals composition, temperature, and motion. Kirchhoff's laws describe continuous, emission, and absorption spectra. Electron transitions between energy levels produce element-specific lines. Helium was discovered in the Sun's spectrum in 1868. Payne-Gaposchkin showed stars are mostly hydrogen and helium. The Doppler effect reveals motion: blueshift toward, redshift away. Doppler wobbles revealed the first exoplanet around a Sun-like star in 1995.
Flashcard
Analysing light by wavelength
Check your understanding
A star's spectral lines are all shifted toward longer wavelengths. What does this tell us?