Emission Mechanisms of Continuous Radiation
In the early days of radio astronomy the receiver sensitivities restricted measurements to the few hundred megahertz range. At such relatively low frequencies the resolving power of the available radio telescopes was low. Initially only very few of the discrete sources could be identified with objects known from the optical region of the spectrum. Further investigations showed an increase in the number of sources with decreasing source flux density and gave the distribution of sources in the sky. It was then concluded that there are two different families of sources: galactic sources, concentrated towards the galactic plane and extragalactic sources distributed more or less uniformly in space. The unresolved, spatially continuous radiation belongs to the galactic component. In addition, there is the 2.7K thermal background radiation which is cosmological in origin.
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