A spectrum is the rainbow-like spread of light that occurs when starlight or other electromagnetic radiation is separated into its component wavelengths, much like a prism splitting white light into colors. This powerful astronomical tool acts as a cosmic fingerprint, revealing the secrets hidden within distant objects across the universe.</p><p>Each chemical element absorbs and emits light at specific wavelengths, creating unique patterns of bright and dark lines called spectral lines. When astronomers analyze these patterns, they can determine what stars and galaxies are made of, even from billions of light-years away. For example, the prominent hydrogen lines at 656.3 nanometers (red) and 486.1 nanometers (blue-green) immediately reveal hydrogen's presence in stellar atmospheres.</p><p>Spectroscopy also unveils motion through the Doppler effect—spectral lines shift toward blue wavelengths when objects approach us and toward red when receding. This technique enabled Edwin Hubble's groundbreaking 1929 discovery that galaxies are moving away from us, proving the universe is expanding.</p><p>Temperature affects spectral line intensity and distribution, allowing astronomers to measure stellar surface temperatures ranging from 2,000K for cool red dwarfs to over 50,000K for hot blue giants. Modern spectrographs can detect magnetic fields, atmospheric compositions of exoplanets, and even dark matter's gravitational effects, making spectroscopy astronomy's most versatile detective tool.