Continuous Spectra Mechanism
In addition to emitting discrete line radiation, the bright-line spectra of a nebula emits a characteristic continuum. The physical mechanisms which are involved in the production of a nebular continuum are as follows:
(a) Recombinations of electrons on discrete levels of hydrogen and to a lesser degree of helium, i.e., because of its lower abundance helium gives only a minor contribution.
(b) Free-free transitions wherein kinetic energy is lost in the electrostatic field of the ions. The thermal radiation from these free-free transitions is observed particularly in the radio-frequency region since these transitions become more important at lower frequencies.
(c) The 2-photon emission is produced by hydrogen atoms cascading from the 2s level to the ground level. The two-photon emission in hydrogen can be expressed as ν1 + ν2 = νLy between the series limits. The recombination spectra decrease as the rate of e-hνkT (where h is Planck's constant,ν the light frequency, k is Boltzmann's constant, and T is the nebula temperature) and it has a maximum approximately halfway between the origin and the Ly. Besides the above, there are other possibilities for contributions to the nebular continuum, namely, electron scattering, fluorescence, and H-emissions. However, the contributions from these do not appear to be especially significant.
The most important feature that is observed in the continuum is the jump, referred to as the Balmer Jump, at the limit of the Balmer series which is produced by the recombination of electron and ions in the n = 2 level of hydrogen. A smaller jump has also been observed at the Paschen limit. The spectral quantities, as well as angular diameter, surface brightness, relative brightness of the principal emission lines, and at times the brightness of the central star are, by and large, readily measurable. Due to this fact, significant contribution can be made to the cosmic abundances as well as to galactic structure.
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Stanley J. Czyzak