Emission lines are brilliant, razor-sharp spikes of light that appear at precise wavelengths in a spectrum when atoms or ions release energy by emitting photons. Think of them as cosmic fingerprints—each element produces its own unique pattern of emission lines, allowing astronomers to identify the chemical composition of distant stars, nebulae, and galaxies from billions of light-years away.</p><p>This phenomenon occurs when electrons in atoms absorb energy and jump to higher energy levels, then cascade back down, releasing photons at specific wavelengths characteristic of each element. Hydrogen's famous red emission line at 656.3 nanometers (H-alpha) creates the stunning crimson glow of star-forming regions like the Orion Nebula. Sodium produces distinctive yellow-orange lines at 589.0 and 589.6 nanometers, easily spotted in street lights and stellar atmospheres.</p><p>The systematic study of emission lines began in the 1860s when Gustav Kirchhoff and Robert Bunsen developed spectroscopy, revolutionizing astronomy by enabling chemical analysis of celestial objects. This breakthrough led to the discovery of helium in the Sun's spectrum in 1868—before it was found on Earth! Today, emission line spectroscopy remains fundamental to astrophysics, helping us measure stellar temperatures, determine galactic distances through redshift, and even hunt for exoplanets by detecting minute spectral changes in starlight.