X-ray observations and the cosmic distance scale

The physics of hot gas in the cores of distant clusters of galaxies can be studied in considerable detail through a specific combination of X-ray, microwave and optical observations. In principle, the X-ray luminosity of the gas can be calculated from these data, providing a measure of the cluster distance. These ideas and their application to Abell 2218, the only cluster for which even marginally suitable data yet exist are examined.