An investigation of the diffuse gas in the interstellar medium

This thesis considers the nature of the diffuse gas in the interstellar medium. In Chapter 3, the ROSAT EUV/X-ray spectra of 1987 late-type stars are studied in detail so that the absorption along the line of sight can be determined. Using two-colour plots, it is shown that the data are best described by two temperature coronal models, although no single model provides an adequate fit for all the sources.;These data are combined with Hipparcos parallaxes and a further 490 sources from other published data sets in Chapter 4 to investigate the distribution of cool gas within 400 pc of the Sun. Using a spherical harmonic analysis it is shown that the best fit is obtained for a non-spherical cavity model in which the Solar System resides in a region of space with a density typically between 0.045cm-3 -0.08 cm-3. The average radius of the cavity is 60 pc although it varies between 29-280 pc with sky direction.;Chapters 5 and 6 consider the X-ray emission from hot components of the diffuse gas. It is shown that spatial fluctuations with a magnitude of 1=4% are observed across the sky and that they cannot be explained by source confusion or background subtraction errors alone. A spectral analysis indicates that they are possibly caused by 'clumps' of gas in the Galactic halo, although the possibility that they are caused by differential absorption across the field of view cannot be ruled out.;In Chapter 7, the techniques used to calibrate the CCD's to be flown on the CUBIC satellite are presented. A combination of laboratory measurements and semiconductor modelling are used to produce response matrices for post launch data analysis.