In 1802 William Hyde Wollaston (1766-1828) observed some dark lines in the solar spectrum and thought they were boundaries between the primary colours. In 1817 Joseph von Fraunhofer (1787-1826) saw that the colour change across the spectrum was in fact continuous and observed some six hundred dark lines. Starting at the red end of the spectrum he labelled the nine most prominent lines with the letters A, B, C, D…, a method still in use today.
In 1854 Robert Wilhelm Bunsen (1811-99) was analysing the colours given off by chemicals heated to incandescence and using coloured glass to distinguish between shades. Gustav Robert Kirchhoff (1824-87) joined this research and suggested that a more precise way of testing the colour of light would be to disperse it with a prism. In 1859 they were able to announce that chemical elements have characteristic bright spectral lines, called emission lines. From the observation of particular bands of bright lines it was therefore possible to identify the chemical element producing it.
Kirchhoff and Bunsen went on to find that the two yellow lines of sodium coincided with two dark lines of the solar spectrum. They concluded that sodium was present in the Sun’s atmosphere. The dark lines of the solar spectrum were called absorption lines because the bright light seemed to have been removed from them (they actually relate to jumps of electrons between the energy levels in atoms).
Julius Plucker (1801-68) studied the spectrums of rarefied natural gases contained in narrow glass tubes and subjected to an electrical discharge. According to one of his contemporaries, he was the first to see the three lines of the hydrogen spectrum, which a few months after his death were recognised in the spectrum of the solar protuberances.
In 1861 Anders Jonas Angstrom (1814-74) began an extensive study of the solar spectrum, confirming the presence of hydrogen in the Sun. In 1868 he published an atlas of the solar spectrum and much improved wavelengths for over one thousand spectral lines. The unit of measurement he introduced for small wavelengths (10-10 metre) was named the angstrom in his honour.
In the 1860s William Huggins (1824-1910) using spectral analysis, showed the stars to be composed of known elements occurring on Earth and in the Sun. In 1864 there were nebulae that astronomers had not been able to resolve into stars. He discovered a bright nebula in the constellation Draco, whose spectra could not be identified as of any element known at the time. He concluded correctly that this nebula was a mass of glowing gas. In 1868 he observed that wavelengths of the spectral lines of Sirius were shifted towards the red end of the spectrum. He realised that this must be due to the Doppler Effect and was able to infer that Sirius was moving away from the Sun at twenty-five miles (≈40 km) per second. He was among the first to use photography to ‘collect’ light. The long-time exposure photograph allows the light from faint objects to collect, resulting in a much brighter image.
In 1868 Pierre Jules Cesar Janssen (1824-1907) went to India to study a total solar eclipse. During the eclipse he analysed the solar spectrum and discovered a yellow line he did not recognise. He reported this to Joseph Norman Lockyer (1836-1920) who attributed it to a new element later named helium, and element discovered on Earth in 1895 by William Ramsay (1852-1916) who isolated it in the laboratory.
Father Pietro Angelo Secchi (1818-78) made the first systematic spectroscopic survey of the heavens and produced a catalogue of more than 4000 stars. By 1868 he had created a spectral classification of stars that lasted until the Harvard classification was introduced in the late 1890s.
In 1872 Henry Draper (1837-82) made the first photograph of a stellar spectrum (Vega). His widow financed the massive photographic stellar spectra survey carried out by the Harvard College Observatory (est.1839) eventually published as the Henry Draper Catalogue of Stellar Spectra (1918-24).
In 1888 Hermann Carl Vogel (1841-1907) detected Doppler shifts in stellar spectra which enabled him to measure reliable line-of-sight (‘radial’) velocities for the first time. The next year he discovered that Algol and Spica were spectroscopic binary stars (i.e. binary stars that cannot be resolved visually or photographically), and derived their masses and orbits.
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