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The Fraunhofer lines are typical spectral absorption lines. Absorption lines are narrow regions of decreased intensity in a spectrum, which are the result of photons being absorbed as light passes from the source to the detector. In the Sun, Fraunhofer lines are a result of gas in the Sun's atmosphere and outer photosphere. These regions have ...
Absorption lines for air, under indirect illumination, so that the gas is not directly between source and detector. Here, Fraunhofer lines in sunlight and Rayleigh scattering of this sunlight is the "source." This is the spectrum of a blue sky somewhat close to the horizon, looking east with the sun to the west at around 3–4 pm on a clear day.
The line is of great interest in Big Bang cosmology because it is the only known way to probe the cosmological "dark ages" from recombination (when stable hydrogen atoms first formed) to the reionization epoch. After including the redshift range for this period, this line will be observed at frequencies from 200 MHz to about 15 MHz on Earth. [21]
Helium has a diffuse series of doublet lines with wavelengths 5876, 4472 and 4026 Å. Helium when ionised is termed He II and has a spectrum very similar to hydrogen but shifted to shorter wavelengths. This has a diffuse series as well with wavelengths at 6678, 4922 and 4388 Å.
Arno Bergmann found a fourth series in infrared in 1907, and this became known as Bergmann Series or fundamental series. [14] In 1896 Edward C. Pickering found a new series of lines in the spectrum of ζ Puppis. This was believed to be the sharp series of hydrogen. In 1915 proof was given that it was actually ionised helium - helium II. [15] [16]
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[4] [5] Fowler managed to produce similar lines from a hydrogen–helium mixture in 1912, and supported Pickering's conclusion as to their origin. [6] Niels Bohr , however, included an analysis of the series in his 'trilogy' [ 7 ] [ 8 ] on atomic structure [ 9 ] and concluded that Pickering and Fowler were wrong and that the spectral lines ...
The Starliner test flight, the first with a crew on board, was launched on June 5 with a known helium leak in the propulsion pressurization system. During approach to the space station the next ...