Title: The Ratio of Total to Selective Extinction and the Distance to the Galactic Centre
Abstract: A method for determining R , the ratio of total to selective extinction, is discussed. It is shown that reliable distances, expressed as a fraction of the distance ρ 0 to the galactic centre, can be calculated from radial velocities and a model for the differential rotation of the Galaxy when care is taken to select suitable stars. A measure of the total extinction can be derived when the apparent distance modulus is known. From the slope of a graph of this extinction against colour excess for many stars, an average value of R is determined; the intercept gives ρ 0. In this paper |$R\,={A}_{V}/{E}_{B-V}$| on the UBV colour system. From a total of 180 OB stars, Cepheids and galactic clusters R is found to be |$3.3_{-0.5}^{+0.7}$| and ρ 0 lies in the range 8–10 kpc.