absorption-line spectrum

Spread the Sun's light through a prism with enough care and the smooth rainbow turns out to be flawed: it is crossed by hundreds of fine dark lines, like missing threads in a coloured fabric. Each dark line is a colour that has gone quietly missing. This is an absorption-line spectrum, and it is the most common kind of spectrum stars show.

It forms when light with all colours — a continuous rainbow from a hot dense source — shines outward through a cooler, thinner gas on its way to us. Atoms in that gas grab photons of exactly their special colours to lift their electrons up a level, robbing the beam of just those wavelengths. The light that emerges is the original rainbow minus a precise set of colours, so we see dark gaps sitting right where that element would have emitted bright lines.

This is why a star's spectrum is mostly a smooth glow scored with dark lines: hot dense gas deep inside makes the rainbow, and the star's cooler outer atmosphere stamps its chemical fingerprint into it as dark absorption lines. By matching those gaps to laboratory patterns, astronomers read off which elements float in a star's atmosphere — the first time anyone learned what the stars are made of.