Interstellar dust average

Several different types of this dust are distinguished by astronomers. On average, interstellar dust resides in widely separated diffuse clouds. But there are also dense regions of gas and dust into which little ultraviolet radiation can penetrate, thereby providing an environment for the formation of complex molecules these are referred to as molecular clouds. Clouds of particles expelled by cooler stars into the regions around them are called circumstellar... 

If all the matter in our galaxy, which is about evenly divided between the stars and the regions between them, were uniformly distributed, its average density would be about 6 X 10 24 g/cm3. The solid component of the interstellar medium composes about 5% of its mass. But despite its low average density—about 1.5 X 10-26 g/cm3—interstellar dust has an important effect on the distribution of electromagnetic radiation in our galaxy. 

The average lifetimes of dust grains in the ISM of about 0.5 Gyr have to be compared with a turnaround time of about 2.5 Gyr for the matter cycle between stars and the ISM, which would result in a small depletion S 0.8 of the refractory elements in the ISM into dust, if depletion of the refractory elements in the returned mass from stars was strong and if no accretion of refractory elements onto dust occurred in the ISM. This clearly contradicts the high observed depletion in the ISM. Hence, most of the interstellar dust is formed in the ISM and is not stardust (Draine 1995 Zhukovska et al. 2008). The most likely place for dust growth in the ISM is in the dense molecular clouds (Draine 1990), but the processes responsible for growth are presently unknown. 

Jessberger E. K., Kissel J., Fechtig H., and Krueger F. R. (1989a) On the average chemical composition of cometary dust. In Evolution of Interstellar Dust and Related Topics, New Holland, New York, 455p. 

To some chemists, the reactions of greatest interest are happening very far away, in the interstellar dust clouds that lie between the stars. The region is dark and cold, with an average temperature of 10 kelvins that s almost 500 degrees below zero on the Fahrenheit scale. 

Most molecules observed to date in interstellar space can be dissociated by UV radiation of wave lengths longer than 912 A. In fact, their average lifetimes in interstellar space are <100 yr, unless they are protected by a dust layer (Section IV. E). This, and the fact that surface reactions on dust grains play an important role in the formation of interstellar molecules (see Section IV. 

Interstellar matter accounts for about 10% of the total mass of the Galaxy. It forms a flat layer about the galactic plane and takes part in the general rotation of the stars around the center of the Galaxy. The average density of the interstellar gas in the galactic plane is between 0.3 and 0.7 atoms cm-3. Dust particles of typical size 0.02 ju to 0.15 jU but of unknown composition are well mixed with the interstellar gas. H and 4He account for 98% of the total mass of the gas. 

The space between the stars. The interstellar matter that occupies this space constitutes several percent of the Galaxy s total mass and It Is horn this matter that new stars are formed. The matter Is primarily hydrogen, in which a number of other molecules and radicals have been detected, together with small solid dust grains. On average the density of matter in interstellar space Is about 10 hydrogen atoms per cubic metre, but the gas Is not uniformly distributed, being clumped into interstellar clouds of various sizes and densities. 

The kinetics of H2 formation (and other surface reactions) via the Langmuir-Hinshelwood (diffusive) mechanism can be treated by rate equations, as in Eq. (1.52), or by stochastic methods.There are two main objections to the former approach it does not handle random-walk correctly and it fails in the limit of small numbers of reactive species. The latter objection is a far more serious one in the interstellar medium because dust particles are small, and the number of reactive atoms and radicals on their surfaces can be, on average, less than unity. Nevertheless, with rare exceptions, the few large models of interstellar chemistry that include surface processes as well as gas-phase chemistry do so via the rate equation approach, so we discuss it here. In the treatment below, we do not use the ordinary units of surface chemistry — areal concentrations or mono-layers — but instead refer to nmnbers of species on the mantle of an individual but average grain. Numbers can be converted to bulk concentrations, as used in Eq. (1.52), by multiplication by the grain number density n. ... 

The space between the stars is not empty, but contains highly rarefied gas and solid particles (dust). The density and temperature, and to some extent the composition, of this interstellar material are highly variable from place to place in our galaxy. On the average, the interstellar gas contains about one hydrogen atom per cubic centimeter, although it can range from 0.01 or less to more than 10 (in dense molecular clouds). It is this interstellar material from which new stars are formed. 

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