The dark lines in the solar spectrum, noted by Wollaston in 1802, were first mapped systematically by the physicist Joseph Fraunhofer. By 1817 Fraunhofer had described hundreds of lines and bands, the principal ones to which he assigned letters A, B, C and so on, which are used even today. Furthermore, he believed that the lines were caused by the nature of sunlight and not a result of diffraction or some other external factor. Brewster, in 1833, using better equipment, showed that at least some of the lines were due to absorption by the Earth’s atmosphere. Fraunhofer proceeded to turn his spectroscope on to the Moon, Venus, Mars and certain bright stars and was impressed by the appearance in most of these of a strong line in the orange region, the D line, a line which also appeared in the spectrum of a sodium flame.

Kirchoff attributed the solar FRAUNHOFER LINES to the absorption of the continuous spectrum produced by the photosphere by vapours in the extensive solar atmosphere, which he associated at the time with the corona. He suggested that the photosphere was a hot, incandescent liquid. The region in the solar atmosphere where the reversal of the lines was supposed to occur was called the reversing layer. Kirchhoff’s theory implied that the solar atmosphere must be at a very high temperature, since the metals had vaporized, with the photosphere being at a higher temperature still since the lines appear in absorption, which meant that the temperature of the Sun must increase inwards. The French astronomer H.Faye proposed a new solar theory in 1865 on the basis of these ideas. Faye considered the entire mass of the Sun to be gaseous, with the heat reaching the outside by means of ascending and descending convection currents.

The study of the solar spectrum underwent rapid development. Kirchhoff identified many lines with terrestrial elements such as sodium, iron, and calcium. S.P.Langley developed an easy method of distinguishing the intrinsic solar lines from the terrestrial ones by comparing spectra from opposite limbs of the Sun. The solar lines are displaced due to solar rotation, a phenomenon not affecting the terrestrial lines. This effect, an example of the Doppler shift, had been described in a general way by Christian Doppler in 1842; in 1848 Fizeau had pointed out that spectral lines could be used to measure velocities. A systematic study of spectra from different parts of the Sun by Sir Norman Lockyer revealed that sunspots contained more numerous and stronger lines than the photosphere; this indicated that spots were regions of lower temperature. Apart from these specific discoveries, more extensive studies of the lines were undertaken. A.J.Angstrom published the wavelengths of 1000 lines; an important identification by him and Thalin was that of hydrogen. This type of work culminated with the publication between 1886 and 1895 of the tables of the American physicist, H.A.Rowland, which provided the wavelengths of 14 000 solar lines from the ultraviolet to the red.

During the late nineteenth century, the application of spectroscopy and photography at a group of total eclipses led to rapid advances in solar physics and particularly on the nature of the outer solar atmosphere. In the 1868 eclipse, bright-line spectra of prominences were obtained which definitively showed that they belonged to the Sun. Janssen and Lockyer independently developed methods for studying prominences out of eclipse. Lockyer noted that prominences emanated from a gaseous envelope, which he called the chromosphere, close to the solar surface. At the 1869 eclipse, the first good photographs of the outer layers of the Sun were taken. W.Harkness discovered the corona’s continuous spectrum, together with a single bright line in the green region of the spectrum. In 1868 Lockyer found that the yellow-orange line emitted by prominences could not be identified with any known element and called the unknown element responsible for it helium. He maintained that it was a new element, but general opinion held it to be the line of a familiar element under exceptional conditions of excitation. In 1895, the British chemist, Sir William Ramsey, isolated helium for the first time. Another important result of the 1869 expedition was the observation by the American astronomer C. A. Young of the flash spectrum. The eclipses that followed continued to reveal many details of the structure and chemical com¬position of the chromosphere, prominences and corona.

Modern research into solar physics can be said to date from the invention in 1891 of the
‘spectroheliograph’ by the American astronomer G.E.Hale and the independent invention of a very similar instrument, the spectral velocity recorder by the French astronomer, H.Deslandres. Both instruments relied on constructing a monochromatic image of the chromosphere. They differed in that Male’s did a continuous scan of a very narrow wavelength band, whereas Deslandres’ isolated a much wider band and moved discontinuously. The two astronomers later became involved in a dispute over the priority of the instrument! In 1905 Hale, sup¬ported by the Carnegie Institution of Washington, set up a large solar observatory on Mount Wilson. His subsequent work revealed, among other things, the existence of the magnetic fields of sunspots and later a general magnetic field about the Sun. The study of solar magnetism begun by Hale provided much of the currently accepted information on the Sun.

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