Dusting the galaxy.

Cosmic dust is one of the two major inhabitants of interstellar space in our and other galaxies (gas clouds are the other). In visible light the dust manifests its presence in a negative, subtractional way by absorbing light and so dimming and reddening the appearance of stars. However, the dust scatters X-rays and so provides halos for the images of point X-ray sources belonging to our galaxy. By studying those halos, Christopher W. Mauche and Paul Gorenstein of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., have deduced something about the size of the dust grains and their density and composition.

Using observations of the Imaging Proportional Counter on the Einstein Observatory satellite, they found first that the intensity of X-ray halos represents an amount of dust consistent with what astrophysicists had calculated from the observed absorption of visible light. In a paper submitted to the ASTROPHYSICAL JOURNAL, they report that the halos cannot be caused by dust grains of a uniform size; there must be a certain distribution of sizes. Theorists have proposed two mathematical schemes for such a distribution, the Mathis-Rumpl-Nordsieck (MRN) formula and the Oort-van de Hulst formula. Although both formulas could account for the halos, the totality of evidence makes Mauche and Gorenstein favor the MRN, which is basically a power law distribution, that is, the number of grains of a given size is proportional to some power of that size. The average size of the grains is about 0.1 micrometer (1/10,000 of a millimeter) and their density about 1 grain per hundred cubic kilometers (10.sup.-12 per cubic centimeter).

Chemically there are two components: graphite grains ranging from 0.005 to 1 micrometer and "silicate" (which could be enstatite, olivine, silicon carbide, iron or magnetite) grains ranging from 0.025 to 0.25 micrometer. From all this Mauche and Gorenstein could calculate that approximately all of the silicon, magnesium and iron and 60 percent of the carbon in the interstellar medium is locked up in dust grains. These figures correspond, they say, to what astrophysicists can calculate independently from the observed composition of the interstellar gas.