The Crab Nebula as a Radio Source.

The optical continuous spectrum of the Crab nebula is rediscussed to ascertain whether the electron temperature in the amorphous mass can be very high. Table 1 contains revised data. The level of ionization maintained by electron impacts is given in Tables 2 and 3. The optical radiation might persist even without a central star. Certain nuclear reactions in a "collapsed" star might produce excess Li and C; broadened lines of ionized carbon should be searched for in the spectrum of the amorphous mass. The thermal emission at radio frequencies is computed on the basis of Elwert's evaluation of the logarithmic term in the free-free opacity. The Crab nebula is found to be optically thick (Table 1) below 100 Mc. It is not possible to reconcile the observed radio flux from Taurus A with the optical flux at any temperature; the thermal eniission is about one-ninetieth of the observed radio flux. Any nonthermal process, like those in other gaseous "point sources," will have its energy distribution changed by the opacity of the surrounding ion-electron cloud. Models in which non- thermal sources suffer absorption prove to have flattened energy distributions (Figs. 1 and 2) at low frequencies, as does the radio source in the Crab nebula. The radio-frequency opacity of other ionized hydrogen regions is found to be appreciable, and distant nonthermal sources seen through bright H U regions will also have flattened spectra. This opacity is proportional to the "emission measure" of the region; the Orion nebula, planetary nebulae, and long paths through weak H II regiolis are optically thick. The Orion nebula should be detectable as a thermal emitter at high frequencies; H II regions toward the galactic center may also be detectable in a narrow range of galactic latitude.