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Galactic Noise

Nature volume 162, page 249 (14 August 1948) | Download Citation

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Abstract

R. v. D. R. WooLLEY has discussed in a recent paper (Mon. Not. Roy. Astro. Soc., 107, 3 ; 1948) the theory of the origin of the galactic radiation. Since radio workers consider it short-wave radiation and spectroscopists regard it as very long-Wave radiation, it is undesirable to refer to it by a name embodying any idea of wave-length, and the appropriate term ‘noise' has been adopted since it can be heard on the earphones. Eddington discussed the temperature of interstellar space in "The Internal Constitution of the Stars", and estimated it at about 10,000° ; but later he suggested that the absorption in space of ultra-violet quanta Would reduce the temperature. (By temperature he meant the parameter appearing in the Maxwellian velocity distribution of the free electrons.) Woolley investigates the problems of temperature and the degree of ionization in interstellar space, and concludes that if the noise is due to free-free transitions, it must certainly come from hot regions in interstellar space and almost certainly from regions where the density is above the average. In these circumstances, if ‘cold' regions, that is, regions of low hydrogen ionization, do not contribute appreciably to the noise, then an observed amount of galactic noise from a particular part of the sky implies a minimum bright hydrogen emission from that part. If observation fails to reveal this bright hydrogen emission, the only conclusion is that galactic noise is not due to free-free transitions. If the noise is due to free-free transitions, it is shown that the minimum number of Hα quanta received per sq. cm. per second per unit solid angle of the sky is within reach of observation with modern equipment.

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https://doi.org/10.1038/162249a0

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