Spectra and Abundances

Abstract

One of the important characteristics of individual galaxies is the abundance of the different chemical elements. In general, galaxies of large mass, such as our Galaxy and the Andromeda galaxy, have abundances of the heavier elements that are high, while low mass galaxies, such as the Magellanic Clouds and dwarf galaxies in the Local Group, have abundances that are ten or more times smaller. The chemical abundances in galaxies are frequently referred to as “metallicity”, because for stars the most conspicuous absorption lines are of iron and other metals. The lighter elements are usually measured in emission nebulae, where emission lines of hydrogen, oxygen, and other light elements can be measured. In this chapter we take up, in turn, spectroscopic measurements of stars in M33, stellar photometric evidence of abundances, spectroscopic evidence for abundances determined for emission regions, and evidence for the existence of the relationship between abundances and distance from the center of M33, the “metallicity gradient”.