On the left, we can see emission lines of a Thorium Argon lamp (extracted from http://tdc-www.harvard.edu/instruments/tres/). On the right a small chunk of the spectrum of Proxima Centauri is shown for comparison (Image credits : G.Anglada-Escude). Sharp features like the ThAr lines and the hundreds of narrow absorption features in Proxima's spectrum are ideal to measure accurate Doppler velocities.