The K-alpha lines in the background X-ray spectrum, and the interstellar gas in galaxies

X-ray observations by satellites have detected lines of iron in the emission spectra of X-ray binaries, galaxy nuclei, and clusters of galaxies. It is shown that by measuring the equivalent widths of the K-alpha lines of iron in the background emission spectra, it is possible to determine the total abundance of iron, sulfur, and silicon atoms in any given direction. Alternatively, if the abundances of these elements are known, the total amount of interstellar matter (gas and dust) can be estimated. By observing the diffuse X-rays of external galaxies in the continuum and in the K-alpha lines, it is also possible to assess the time variability of the central X-ray source over the past 30,000 years.