spectroscopy

In 1896, Pieter Zeeman placed sodium flame between magnet poles and watched spectral lines split. The experiment revealed the electron's charge-to-mass ratio and earned the 1902 Nobel Prize in Physics.

Spectroscopy, the analysis of light spread out by wavelength, is the most productive tool in astronomy. From a single beam of starlight it can reveal temperature, composition, speed, rotation, and magnetic field strength. Doppler shifts of spectral lines exposed the expansion of the universe, and the fingerprints of atoms identify every element the light has passed through.

Telescopes can read the chemical fingerprints of atmospheres on planets hundreds of light-years away using transmission spectroscopy — splitting light into a spectrum that reveals which molecules are present. Finding oxygen, ozone, or methane in the right combination could indicate life, but interpreting these signals without false positives is enormously complex.