At wavelengths shorter than 91 nm it is not only the dust grains in the interstellar medium which absorb starlight. At these extreme ultraviolet (EUV) wavelengths the hydrogen atoms themselves are very strong absorbers of radiation by virtue of their ability to be¬come ionized by such energetic photons. It is in the wavelength range just short of 91 nm that the interstellar medium is most opaque. A sensitive experiment aboard the USA-USSR Apollo-Soyuz spacecraft in 1975 was able to see only a handful of celestial objects at these wavelengths, and all of these are comparatively close to the Sun.

At wavelengths around 90 nm the absorption is about 20 magnitudes per parsec, or 10 000 times greater than that at visible wave¬lengths. Shortward of this wavelength the absorption decreases again fairly quickly, but at about 4 nm (corresponding to a photon energy of about 0.3 keV) absorption by other atoms, such as carbon, nitrogen and, especially, oxygen, become important. Although these atoms together number less than 1 per cent of the number of hydrogen atoms, they absorb so much more energy per ionization that it is they, not hydrogen, that are the main cause of opacity in the X-ray region. They greatly hinder observation of X-ray sources, at energies less than about 2 keV.

The X-ray absorption efficiency of atoms like carbon, nitrogen and oxygen is essentially unaffected by whether or not they are incorporated into molecules or dust grains. By measuring the X-ray absorption in any given direction and comparing it with the amount of hydrogen as estimated using the 21-cm line it is possible to check on the abundances of the heavy elements without having to consider the extent to which they are condensed onto dust grains. Early experiments of this kind have tended to confirm previous ideas about the expected heavy element abundances, but more sensitive techniques which will become available soon are expected to indicate some anomalies.

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