Molecular clouds Galaxy

For remote sensing, spectroscopy at THz frequencies holds the key to our ability to remotely sense enviromnents as diverse as primaeval galaxies, star and planet-fonuing molecular cloud cores, comets and planetary atmospheres. 

Dense molecular clouds are too dusty to be studied by optical means, but radio and mm wave observations reveal numerous molecules such as CO, CH2O, HCN, NH3 etc. which give valuable information on isotope ratios across the Galaxy and... 

In recent decades, spectroscopy has revealed that the elemental and isotopic abundances in the galaxy vary with radial position and that the Sun has a somewhat different composition than the molecular clouds and diffuse interstellar medium in the solar neighborhood. For this reason, we can no longer think of the solar system abundances as truly cosmic abundances. 

In this chapter, we have introduced solar system abundances and cosmic abundances of the elements and isotopes and briefly discussed their importance in cosmochemistry. The idea of cosmic abundances has been around for over a century and is rooted in measurements of many different celestial objects. Only in the last 20-30 years has our knowledge of the compositions of the solar system, of stars and molecular clouds in our galaxy, and of other galaxies grown sufficiently that we can now detect clear compositional differences between our solar system and other objects in the galaxy and the universe. 

The molecules found to date are composed of the elements H, C, N, O, Si, S, and Cl with the bulk of the molecules containing H, C, N, and O. The light elements H, D, and He are of cosmological origin and are therefore tracers of the early universe. On the other hand the heavier elements C, N, O,... are produced in stars by the processes of stellar nucleosynthesis. In addition to the most abundant isotopic forms many stable isotopes such as D, 13C, 170, lsO, 15N, 30Si, 33S, and 34S have been detected (see Appendix 1). The detailed determination of isotopic ratios — though often beset with formidable difficulties — has become a useful indicator of the chemical evolution of molecular clouds and the past chemical history of the galaxy. 

Interstellar medium The dust, molecular clouds, and neutral hydrogen that lie between the stars of this galaxy, generally in the plane of the Milky Way, but whose density is highly variable... 

The visible matter of galaxies is concentrated in mainly three components stars, interstellar matter, and stellar remnants. Since the early days of galaxy formation there is a vivid exchange of matter between the stellar component and the interstellar matter. Stars are formed in local concentrations of the ISM, the molecular clouds they live for a certain period of time while burning their nuclear fuels and they die... 

There is convincing observational evidence that the placental interstellar medium (ISM) from which the solar system originated was a dense molecular cloud (Wasserburg et al., 1982 1979). In fact, the recent evidence of the presence of short-lived nuclei in meteorites requires that the free-fall time scale for gravitational collapse (tft) be less than or comparable to the mean lifetime of Al ( 10 yrs), i.e. ttt 4.10 / /n < 10 yrs, which requires nn lOVcc, a value typical of molecular clouds. Since molecular clouds are observed to be a major feature in our galaxy, they constitute a most reliable starting point for the processes that will eventually lead to the formation of stars and planetary systems (Falk and... 

Giant molecular cloud formations, consisting almost entirely of hydrogen, are the most massive objects within galaxies. Gravity eventually causes the hydrogen to compress until it fuses into heavier elements. 

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