Abstract
GALACTIC gas is present in three types of regions. According to Schlüter1, these are: (1) cold dense HI clouds of temperature of the order of 100° K. and density of 10 atoms/cm.3, occupying about 5 per cent of space; (2) these are surrounded by a hot transparent continuous gas at 10,000° K.. density 0.1 atom/cm.3 and pressure 10−13 dyne/cm.2; (3) in addition, about 0.5 per cent of space contains hot dense rapidly expanding H II regions, heated by nuclear energy derived from embedded O or B type stars supernovæ and novæ. The temperature of 10,000° K. of the main continuous component is probably maintained by turbulent and viscous dissipation of rotational galactic shear, which can supply 10−25 erg/cm.3 sec. for 6 × 109 years, and by absorption of stellar radiation. The energy sink is provided by radiation into intergalactic space. The H I regions must have a comparatively short life of some 107 years and must be replenished. Steady population of all ages must develop, in which condensations are born from the hot continuous component by Zanstra-type instability based on strong dependence of cooling of ionized gas on density. Denser eddies of ionized gas continue losing internal pressure by excessive radiative cooling and collapse from the excess pressure of the surround hot gas. After recombination, cooling-rate slows down, but shear energy supply disappears. An ideal spherical region would deform under galactic shear first into an ellipsoid, which would then continue to collapse into an elongated disk of very small thickness. Collisions of H I regions and turbulence of the continuous component combined with galactic shear would tend to disperse such sheets and return their contents to the hot gas, closing thus their cycle of evolution.
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References
Schlüter, Internat. Astro. Union Symposium No. 2, p. 144 (North-Holland Pub. Co., Amsterdam, 1955).
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VAND, V. Formation of Stellar Assciations from Galactic Gas. Nature 184, 441 (1959). https://doi.org/10.1038/184441a0
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DOI: https://doi.org/10.1038/184441a0
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