Analyzing Starlight (Astronomy)

Recall cards on how astronomers read a star's light. Brightness: the difference between luminosity (total energy emitted per second) and apparent brightness (energy reaching us), the inverse-square dimming with distance, and Hipparchus's magnitude scale on which brighter means a smaller number and five magnitudes span a factor of 100. Color: a star's surface temperature sets its color, from over 40,000 K blue-white down to about 2,000 K red, quantified by the UBV color index. Spectra: the temperature-ordered OBAFGKM (plus L, T, Y) classes, their spectral-line signatures, brown dwarfs, and Annie Jump Cannon. And what spectra reveal: composition (about three-quarters hydrogen), radial velocity from the Doppler shift, proper motion across the sky, and rotation and pressure read from the width of the lines.