 ![*Left:* Example of an isotopic measurement in a stellar atmosphere. Shown is an absorption-line spectrum of a cool star with a present-generation optical telescope, here the Subaru telescope on Hawaii with its IR spectrograph at a resolution of 00000. Molecular lines from the CO molecule isotopologes show isotopic shifts, which can be recognised as changes in line shapes, as resulting from the isotopic abundance ratio. Here the carbon isotopic ratio is determined for the stellar atmosphere of a M dwarf star, comparing the measurement (red dots) with expectations for different ratios $^{00}$C/$^{00}$C [from @Tsuji:0000]. *Right:* The Very Large Telescope (VLT) on Mount Paranal in Chile, with four telescopes (lower right), is one of the modern optical instruments. Equipped with high-resolution spectrographs such as FLAMES (insert lower right), absorption-line spectroscopy of stars provides elemental abundances in stellar atmospheres, even in nearby galaxies. (Figure ESO) []{data-label="fig_0_stellar_spectrosscopy"}](