Spectra of dwarf stars in NGC 6752

A series of spectra of dwarf stars in the globular cluster NGC 6752, obtained with the UVES high-dispersion spectrograph at the 8.2-m VLT KUEYEN telescope. Sodium (Na) and Oxygen (O) lines are marked, and the spectra are arranged according to the strength of the Sodium lines, with the strongest at the top. It is obvious that stars with stronger Sodium lines (and therefore with a higher Sodium abundance) have weaker Oxygen lines (and are therefore poorer in Oxygen). Even with UVES, the most powerful high-resolution astronomical spectrograph in the world, exposures of up to 4.5 hours were required to record good spectra of these faint objects (V-mag = 17.2).