Light as Information

Part 7: Real Stellar Spectra

Beyond the Ideal Blackbody

Stars are approximately blackbodies. Real spectra show two features: the overall shape follows a Planck curve (temperature, via Wien’s law), and absorption lines appear as dark notches (composition, via spectroscopy).

The spectrum of Altair (an A-type star) shows the smooth blackbody shape plus sharp absorption dips from hydrogen and other elements — two inference tools in one observation: the curve shape gives temperature, the line positions give composition.

Compare the Sun’s spectrum (approximately continuous, like a blackbody) with a fluorescent bulb (discrete emission lines): the shape tells you the emission mechanism — thermal versus non-thermal.

Doppler Shift: Motion in Spectral Lines

Spectral lines are also speedometers. If a source moves toward or away from us, its lines shift slightly, encoding the line-of-sight velocity — we measure motion using nothing but light.

For speeds much smaller than $c$, the non-relativistic Doppler shift is:

This tiny shift unlocks some of the biggest stories in astronomy: a star’s wobble reveals exoplanets; binary Doppler shifts give orbital speeds and masses; line shifts across a galaxy map its rotation (and point to dark matter); and overall redshift shows the universe is expanding.