Interstellar Matter and Cosmic Radiation

In interstellar space, matter is distributed very unevenly. As already mentioned in section 14.4, some stars are ejecting their matter in form of nebulas of dust and gas. These nebulas contain various elements (mainly H, C, O, Si and others) at temperatures between about 10 and 10 K. Far away from the stars, the density of interstellar matter is of the order of 0.1 atom per cm, mainly H and C. In some regions, however, matter is condensed in the form of big interstellar clouds, the mass of which may exceed the mass of the sun by a factor of 10 or more. Two types of interstellar clouds are distinguished optically transparent, diffuse clouds containing 10 atoms per cm (mainly H, but also some compoimds such as CO or HCFIO) at temperatures of the order of 100 K, and opaque, dense clouds containing lO to 10 molecules per cm (mainly H2, but also a variety of compounds) at temperatures varying between about 10 and 10 K. Densities and temperatures increase from the outer parts to the core of the clouds. 

The present state of knowledge about invisible dark matter indicates that it represents the predominant part (about 90%) of the total mass of the universe. It includes the so-called missing mass. Dark matter is not discernible by any kind of electromagnetic radiation in the region from y rays to radiowaves, but its gravitational effects on other kinds of matter are observable. For example, the rotation of spiral galaxies such as the Milky Way can only be explained if 90% of the matter is invisible in the sense mentioned above. The question of the nature of the dark mass is still open. Various possibilities are discussed matter different from that on the earth (no protons, neutrons and electrons), remnants of the big bang, neutrinos. However, the actual mass of neutrinos is still uncertain. 

An important proportion of the matter in the interstellar space appears in the form of cosmic rays consisting of high-energy particles and photons. The intensity of cosmic radiation (about 10 particles per cm per s) is relatively weak compared with the 

Primary cosmic rays consist mainly of protons ( 90%). Hehum nuclei and electrons ( 5% each), ions heavier than He ( 1%) and y-ray photons are less abundant. Most of the protons have energies of the order of 10 MeV. 

The cosmic radiation incident on the earth is generated in our galaxy. It is effectively absorbed in the atmosphere, and the flux density is reduced from about 20 cm s to about 1 cm s at the surface of the earth. By interaction with the atoms and the molecules in the atmosphere showers of elementary particles are produced, making up the secondary cosmic radiation. Positrons, muons, several kinds of mesons and baryons were first detected in the secondary cosmic radiation. Furthermore, nuclear reactions induced by secondary cosmic radiation lead to the production of cosmogenic radionuchdes, such as T and (section 1.2). 

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