An accurate value of the deuterium/hydrogen ( D/H ) ratio in the local interstellar medium ( LISM ) and a better understanding of the D/H variations with position in the Galactic disk can provide essential information on the primordial D/H ratio in the Galaxy at the time of the protosolar nebula , and the amount of astration and mixing in the Galaxy over time .
Recent measurements have been obtained with UV spectrographs on FUSE , HST , and IMAPS using hot white dwarfs , OB stars , and late-type stars as background light sources against which to measure absorption by D and H in the interstellar medium along the lines of sight .
Recent analyses of FUSE observations of seven white dwarfs and subdwarfs provide a weighted mean value of D/H = ( 1.52 \pm 0.08 ) \times 10 ^ { -5 } ( 15.2 \pm 0.8 ppm ) , consistent with the value of ( 1.50 \pm 0.10 ) \times 10 ^ { -5 } ( 15.0 \pm 1.0 ppm ) obtained from analysis of lines of sight toward nearby late-type stars .
Both numbers refer to the ISM within about 100 pc of the Sun , which samples warm clouds located within the Local Bubble .
Outside of the Local Bubble at distances of 200 to 500 pc , analyses of far-UV spectra obtained with IMAPS indicate a much wider range of D/H ratios between 0.8 to 2.2 ppm , providing information on inhomogeneous astration in the Galactic disk .