Spectra of \sim 150 early-type stars in the disk of the Milky Way recorded by FUSE reveal absorption features from interstellar O VI , a tracer of collisionally ionized gases at temperatures near 300,000 K. ( Hotter material is better sensed by its diffuse x-ray emission . )
The FUSE survey indicates that hot gas in the plane of the Galaxy yields an average density n ( { O~ { } VI } ) = 1.7 \times 10 ^ { -8 } { cm } ^ { -3 } , a value consistent with earlier results from the Copernicus satellite , but at great distances there is a large dispersion in the measurements from one line of sight to another .
This dispersion indicates that O VI absorbing regions probably have a broad distribution of sizes , perhaps giving a power-law behavior for column densities .
Comparisons of O VI with other Li-like ions should give some guidance on the nature and origin of the O VI-absorbing regions , but poor correspondences in the velocity profile shapes will complicate the interpretations .