About Astronomical Spectroscopy:

Astronomical spectroscopy started when Isaac Newton first observed sunlight dispersed by a prism. He saw a rainbow of colour, and possibly may have observed absorption lines. These dark bands which appear throughout the solar spectrum were first later described by Joseph von Fraunhofer. Most stellar spectra share these two dominant features of the sun's spectrum: emission at all wavelengths across the optical spectrum (the continuum) with many discrete absorption lines superimposed on top. Fraunhofer and Angelo Secchi were among the pioneers of spectroscopy of the sun and other stars. Secchi is particularly noted for classifying stars into spectral types, based on the number and strength of the absorption lines in their spectra. Later the origin of the spectral types was found to be related to the temperature of the surface of the star: particular absorption lines can be observed only for a certain range of temperatures; because only in that range are the involved atomic energy levels populated. The absorption lines in stellar spectra can be used to determine the chemical composition of the star. Each element is responsible for a different set of absorption lines in the spectrum, at wavelengths which can be measured extremely accurately by laboratory experiments. Then, an absorption line at the given wavelength in a stellar spectrum shows that the element must be present. Of particular importance are the absorption lines of hydrogen (which is found in the atmosphere of nearly every star); these are known as Balmer lines. In 1868, Sir Norman Lockyer observed strong yellow lines in the solar spectrum which had never been seen in laboratory experiments. He deduced that they must be due to an unknown element, which he called helium, from the Greek helios (sun). Helium wasn't conclusively detected on earth until 25 years later.

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