Helium interstellar

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

Sometimes a star explodes in a supernova cast mg debris into interstellar space This debris includes the elements formed during the life of the star and these elements find their way into new stars formed when a cloud of matter collapses in on itself Our own sun is believed to be a second generation star one formed not only from hydrogen and helium but containing the elements formed in earlier stars as well... 

What a storyi Fullerenes formed during the ex plosion of a star travel through interstellar space as passengers on a comet or asteroid that eventually smashes into Earth Some of the fullerenes carry pas sengers themselves—atoms of helium and argon from the dying star The fullerenes and the noble gas atoms silently wait for 251 million years to tell us where they came from and what happened when they got here... 

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. 

Helium fusion moves directly from helium to carbon, leaping across the lithium-beryllium-boron trio. These nuclei are not produced in stars. Indeed, they are destroyed there, as a result of their excessive fragility. They are generated in the interstellar medium by collisions between high energy nuclei and protons and helium nuclei at rest, and also by the opposite process which amounts to swapping over target and projectile, as already mentioned. 

Clouds of gas in the interstellar medium are called gaseous nebulas. These nebulas are regions of the interstellar medium with above-average density. The proportions of elements in the interstellar medium conform to the abundances in the table, that is, 90% hydrogen atoms, 9% helium atoms and less than 1% heavier atoms, where these percentages now refer to relative numbers of atoms rather than relative mass. 

This paper summarizes the results of analyses of highly evolved stars with spectral type B or hotter, namely sdB, sdOB and sdO types, CSPN and extremely helium-rich stars. It does not consider white dwarfs since their chemical surface composition is apparently governed by diffusion processes and accretion of interstellar material (Wesemael, 1979 Vauclair et al., 1979 Wesemael and Truran, 1982) and is not linked to their past evolution. Section 2 deals with the positions of the hot evolved stars in the (log Te -log g) plane and their helium to hydrogen ratios. Metal abundances are considered in section 3 and comparisons of stellar evolution calculations with the available data are performed in section 4. 

They were either formed with the progenitor from the interstellar medium, or they were created in it as an adjunct to helium-burning and mixed to the surface. In the latter event, some enrichment of the elements would be expected, although calculations of the evolution of massive stars have indicated that no products from the helium burning core should be convected to the surface (Lamb et al. 1977). 

ASTROCHEMISTRY. Application of radioastronomy (microwave spectroscopy) to determination of the existence of chemical entities in the gas clouds of interstellar space and of elements and compounds in celestial bodies, including their atmospheres. Such data aie obtained from spectrographic study of the light from the sun and stars, from analysis of meteorites, and from actual samples from the moon. Hydrogen is by far the most abundant element in interstellar space, with helium a distant second. 

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