What critical physical information is derived by analyzing the spectral lines in the dispersed light from a galaxy using a spectrograph?

Identifying specific elements present, temperature, and velocity via the Doppler effect.

The process of spectroscopy, where light is dispersed like a rainbow, is central to galactic physics because the resulting spectrum contains detailed information encoded in dark or bright lines. These spectral lines function as unique fingerprints for the material emitting or absorbing the light. By matching the position of these lines to known atomic transitions, scientists can identify precisely which elements exist within the observed gas cloud or galaxy. Furthermore, shifts in the position of these lines allow for the measurement of the object's velocity relative to the observer, quantified using the Doppler effect, indicating whether the object is receding or approaching.