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in astronomical spectroscopy, bright emission lines in the spectra of certain nebulae (H II regions), not observed in the laboratory spectra of the same gases, because on Earth the gases cannot be rarefied sufficiently. The term forbidden is misleading; a more accurate description would be “highly improbable.” The emissions result from electrons in long-lived orbits within the...
...were first detected as unresolved sources in surveys conducted during the 1950s by radio astronomers in Cambridge, Eng. Optical photographs subsequently taken of their spectra showed locations for emission lines at wavelengths that were at odds with all celestial sources then familiar to astronomers. The puzzle was solved by the American astronomer Maarten Schmidt, who announced in 1963 that...
...of unresolved stars (as are galaxies) or glowing gas. Stars radiate at almost all wavelengths, with perhaps dark absorption lines superimposed, while hot, transparent gas clouds radiate only emission lines at certain wavelengths characteristic of their constituent gases. In 1864 observation of the spectrum of the Orion Nebula showed bright emission lines of glowing gases, with...
in nebula: Reflection nebulae )...is of this type; it was discovered in 1912 that the spectrum of this nebula mimics the absorption lines of the nearby stars, whereas bright nebulae that emit their own light show characteristic emission lines quite unlike stars. The brightest reflection nebulae are illuminated by B-type stars that are very luminous but have temperatures lower than about 25,000 K, somewhat cooler than the...
in nebula: Chemical composition of diffuse nebulae )The composition of diffuse nebulae can be estimated by relating the strengths of the emission lines found in their spectra to the numbers of atoms producing them. Great strides have been made in calculating the necessary atomic properties. The principal difficulties in determining the abundances of elements from nebular emission lines are (1) the estimation of the nebular temperature, which is...
...A potential difference of 10–100 kilovolts is maintained between cathode (the negative electrode) and anode (the positive electrode). The X-ray spectrum emitted by the anode consists of line emission and a continuous spectrum of radiation called bremsstrahlung radiation. The continuous spectrum results from the violent deceleration of charges (the sudden “braking”) of the...
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