Interstellar dust clouds

An interstellar dust cloud containing aligned particles may be looked upon as a linearly birefringent (and possibly linearly dichroic) medium (van de Hulst, 1957, p. 58) the cloud acts like a retarder. We showed at the end of Section 2.11 that linearly polarized light becomes circularly polarized upon transmission by a retarder. The first clear evidence for this kind of polarization mechanism was reported by Martin (1972), where light from the Crab Nebula was the source of linearly polarized incident light. 

In the past 10 years a large number of organic molecules have been found in interstellar dust clouds mostly by emission lines in the microwave region of the spectrum (for a summary see Ref. 38). The concentration of these molecules is very low (a few molecules per cm3 at the most) but the total amount in a dust cloud is large. The molecules found include formaldehyde, hydrogen cyanide, acetaldehyde, and cyanoacetylene. These are important prebiotic molecules, and this immediately raises the question of whether the interstellar molecules played a role in the origin of life on the earth. In order for this to have taken place it would have been necessary for the molecules to have been greatly concentrated in... 

Various forms of molecular carbon, from ions to radicals, have been detected in the diffuse interstellar medium (ISM) using electronic, rotational, and vibrational spectroscopies (Henning and Salama 1998 Snow and Witt 1995). Discrete absorption and emission bands seen toward diffuse interstellar clouds indicate the presence of numerous two-atom molecules such as CO, CN and C2. In addition to these interstellar features, a large family of spectral bands observed from the far-UV to the far-IR still defies explanation. Currently, it is the general consensus that many of the unidentified spectral features are formed by a complex, carbonaceous species that show rich chemistry in interstellar dust clouds (Ehrenfreund... 

Before 1985, six crystalline forms of carbon were known two forms of graphite, two forms of diamond, and chaoit and carbon (VI) discovered in 1968 and 1972, respectively. In addition a number of almost pure amorphous forms exist, such as polyacetylene (7.60) and cumulene (7.61) and recently a number of interesting nanostructured forms of carbon have been produced (Section 15.8). The year 1985 marked the discovery of the fullerenes, which represent the only truly pure molecular form of carbon, are produced under very extreme conditions as carbon vapour condenses in an atmosphere of an inert gas such as helium. Harold Kroto s interest in this chemistry originated with microwave spectroscopic studies of the atmosphere of stars and interstellar dust clouds. Kroto wanted to try to reproduce in the laboratory spectra of carbon... 

The use of telescopic spectroscopy has revealed the existence of glycine in interstellar dust clouds. Since these clouds amount to huge masses of matter (greater than the total mass of condensed objects such as stars and planets), there must be universal availability of amino acids, even though they are dispersed thinly in the vast volume of space. 

It is well established observationally that stars form in massive interstellar dust clouds. If the Jeans condition is satisfied (cf. Eq. (25)), then parts of the cloud will start to collapse, leading to the release of gravitational energy. Initially, the star will be very luminous but cool, and may be identified with Hot Cores in giant molecular cloud complexes. Low-mass protostars are... 

The recent discoveries of complex organic molecules associated with interstellar dust clouds have, of course, attracted great interest among students of chemical evolution. Inspection of Table IV will reveal several compounds which have been referred to with regard to the syntheses of biologically significant compounds HCN, formaldehyde and cyanoacetylene. It is perhaps unlikely that the processes which have produced these com-... 

X 10 kg. Analyses of interstellar dust clouds and heavenly bodies fotmd on Earth, such as the Murchison meteorite, showed the presence of a variety of important prebiotic compotmds. such as amino acids, ureas, alcohols. aldehydes, purines, polycyclic aromatic hydrocarbons (PAH), and organic acids,"" highlighting the significance of extraterrestrial carbon delivery to Earth. 

There is experimental support for this in the finding that nanometre diamonds are abundant in primitive meteorites and possibly in the interstellar dust clouds [104]. Similarly, growth of graphite in the diamond-stable region during high pressure, high temperature synthesis, and the corollary, an overpressure for nucleation of... 

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. 

According to present-day concepts, our solar system was formed from a huge gas-dust cloud several light years across in a side arm of the Milky Way. The particle density of this interstellar material was very low, perhaps 108-1010 particles or molecules per cubic metre, i.e., it formed a vacuum so extreme that it can still not be achieved in the laboratory. The material consisted mainly of hydrogen and helium with traces of other elements. The temperature of the system has been estimated as 15 K. 

Interstellar dust is also important for the formation and development of stars. Although the dust particle component is only a minor one in ISM, it acts as a cooling agent for collapsing clouds, thus preventing the buildup of an effective thermodynamic counterpressure. 

The consequences of interstellar dust may be seen with the unaided eye. Under good seeing conditions dark patches can be observed in the Milky Way. These dark areas, we now know, do not result from the irregular distribution of stars in our galaxy but rather from the very effective obscuration of starlight by irregular clouds of small particles, the interstellar dust. 

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... 

Nebula A cloud of interstellar dust and gas, which may condense to form stars and even solar systems (compare with Solar Nebula). 

It is reasonable to consider the assumption that life began, somehow, among one of the mixtures of small organic molecules that are produced by abiotic processes. The only natural examples in hand today are the components of meteorites that have fallen to Earth (see Section 5.2.1) and particles returned by the Stardust mission. Spectroscopy has also yielded partial lists of the organic molecules in interstellar space and interplanetary dust clouds. 

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