Reading Starlight: Spectra, Composition, and the Doppler Shift

Hi, I'm Lumi. Starlight looks plain white, but split it with a prism and it fans into a spectrum — a band of colors ordered by wavelength, from short blue (around 400 nm) to long red (around 700 nm). Hidden in that band is a coded message about the object that made it. A hot, dense source like a star's deep layers makes a continuous spectrum, all colors smoothly blended. Its peak wavelength tells us temperature — this is called Wien's Displacement Law (λ_max = b ÷ T, where b ≈ 2.898 × 10⁻³ m·K). Hotter objects peak at shorter wavelengths toward the blue end; cooler ones peak at longer wavelengths toward the red end. That is why an O-type or B-type star, whose blackbody peak falls in the blue or ultraviolet, has a far higher surface temperature than an M-type star whose peak falls in the red or infrared. Now the lines. Atoms of each element absorb and emit only certain exact wavelengths — a fingerprint. When light passes through a cooler gas, that gas removes its special wavelengths, leaving dark absorption lines in the band. A thin glowing gas instead gives off only those wavelengths, making bright emission lines on a dark background. Either way, matching the line pattern to known elements tells us composition — what the object is made of, such as hydrogen or helium. Finally, motion. If the object moves toward us, its whole line pattern shifts to shorter wavelengths — a blueshift. If it moves away, the pattern shifts to longer wavelengths — a redshift. This is the Doppler shift, and the size of the shift reveals the line-of-sight speed. The lines keep their relative spacing, so we still recognize the same elemental fingerprint, just slid along the band. So one spectrum carries three answers at once: composition from the line pattern, temperature from the peak wavelength, and velocity from how far the lines have slid. If a spectrum ever stumps you, start by asking those three questions one at a time.