Copernicus observations of interstellar absorption at Lyman alpha

Column densities N/sub H/ of atomic hydrogen have been derived for 40 OB stars from spectral scans at Lα obtained by the Copernicus (OAO-3) satellite. The stars are all between 60 and 1100 pc away with a range of mean densities n/sub H/ of 0.01 to 2.5 atoms cm -3. For 27 stars without significant amounts of molecular hydrogen, the mean ratio of gas to color excess is =3.6times1021 atoms cm-2 mag-1, and the mean density is =0.12 atoms cm-3. For 10 stars where the molecular hydrogen is at least 5 percent of the total hydrogen, is 5.4times1021 atoms cm -2 mag-1 and is 0.7 atoms cm-3. In this limited set of data, the ratio of gas to color excess in clouds varies from 1 to 3 times the mean outside of clouds. The presence of molecular hydrogen correlates with E (B-V), but the best tracer for H2 is atomic hydrogen. The mean density of the gas for all 40 stars is much smaller than the mean of 0.7 atoms cm-3 obtained from 21-cm observations, because the brightest stars with less than average amounts of matter in the line of sight were selected for observation