Absorption Line

Absorption lines are distinctive dark bands that appear in a spectrum when cooler gas absorbs specific wavelengths of light from a hotter, continuous light source behind it. Think of it like a cosmic fingerprint – each element creates its own unique pattern of dark lines at precise wavelengths, allowing astronomers to identify what's out there in space.</p><p>This phenomenon occurs because atoms absorb photons at exactly the same wavelengths they would emit, lifting electrons to higher energy levels. The absorbed light energy is later re-emitted in random directions, creating dark gaps in the original spectrum at wavelengths like 656.3 nanometers for hydrogen's red line.</p><p>Discovered by Joseph von Fraunhofer in 1814, who catalogued over 500 dark lines in the Sun's spectrum, absorption spectroscopy revolutionized astronomy. Today, astronomers use these stellar "barcodes" to determine a star's chemical composition, temperature (typically 3,000-50,000K for main sequence stars), and motion through Doppler shifts.</p><p>Practical applications include discovering exoplanets when their atmospheres absorb starlight during transit, measuring galactic recession speeds that revealed universal expansion, and determining that stars contain the same elements as Earth. The Hubble Space Telescope routinely uses absorption line analysis to study distant galaxies, revealing that the universe's chemical evolution spans billions of years through this elegant natural spectroscopy.