What chemical compound gives fireflies their glow?

The chemical compound that gives fireflies their glow is luciferin.

In fireflies, luciferin is the light-producing molecule, while luciferase is the enzyme that helps trigger the reaction. Oxygen is also required, and firefly luciferin’s reaction uses ATP and magnesium ions. The resulting unstable intermediate releases energy as visible light rather than mainly as heat.

The glow is a form of bioluminescence. After luciferin is oxidized, the product oxyluciferin enters an energized state. When it returns to its normal state, it emits a photon. Firefly species can produce somewhat different colors because of differences in luciferase structure and the chemical conditions inside their light organs.

A famous historical distinction separates luciferin from luciferase. In 1885, French scientist Raphaël Dubois extracted the heat-stable light-producing substance and the heat-sensitive enzyme from glowing organisms. The names were later applied broadly to related light-producing systems, so “luciferin” is a family term as well as the name commonly used for firefly luciferin.

Firefly luciferin is also valuable in laboratories, where luciferin–luciferase reactions are used to track gene activity and image biological processes.