In the mid-nineteenth century, very little was known about stars other than positions, apparent magnitudes, and, in a few cases distances. The method of determining distance, by trigonometrical parallax, was limited to those stars within a hundred parsecs of the Sun ; thus, even by 1890 fewer than 100 stars had known distances. Spectral analysis did not suffer from this limitation. As long as sufficient light from a star could be collected, much information could be gathered about that star and in the early 1860s these studies were begun by William Huggins and Angelo Secchi. Their different interests characterize the two different aspects of the field which were to develop. Huggins, working with W.A.Miller, was interested in the detailed spectra of stars so as to analyse their chemical composition. In 1864 they published a paper on the spectra of some 50 bright stars, from which they concluded that (stars have a similar chemical composition to the Sun and that J most of these elements – hydrogen, sodium, magnesium, calcium, iron and so on – play an important role in terrestrial physics and organic life. This link with organic life and consequently its evolution a major controversial issue at the time, played an influential role in the discussions on the evolutionary order of the stars. The argument ran that as a species evolves, so must a star, and thus a picture of the linear evolutionary development of stars soon fol¬lowed. The actual sequence was established by the classification of stellar spectra begun by Secchi. In 1868, Secchi published a catalogue of 4000 stars which he divided into four classes according to the appearance of their spectra. The idea that each sequence indicated successive stages in the life of a star was soon taken for granted. Secchi’s classification was achieved by a visual examination of the spectra; the wet collodion process used by photographers at the time was too crude. In the 1870s the development of dry gelatine plates meant that a closer study of the spectra could be made, including a broader band of the spectrum as photographs revealed the ultraviolet region of the star’s radiation. Both aspects of the work that followed – the detailed examination of structure and the large-scale classification and consequent evolution involved a close interaction with developments in the chemical laboratory, where experiments were being made on the varying effects of different conditions on the spectra of gases.

An important by-product of Huggins’ work was the determination of the first line-of-sight velocity of a star. The French physicist, Fizeau had pointed out in 1848 that spectral lines provided very good markers for measurements of the Doppler effect. In 1868, Huggins announced a very small shift in the lines of Sirius, indicating a velocity of recession of 47 kms-1.

Stellar data were accumulating rapidly and, as a result, a more definite picture of stellar evolution. Secchi had classified the stars into four types – white stars like Sirius, yellow stars such as the Sun, orange and reddish stars like Betelgeuse and faint red stars -according to the appearance of their spectra. He thought that his classification represented a temperature sequence, but he was more interested in actual spectral differences.

By the end of the century, there was little doubt of the order of evolution of the four major classes helium stars into Sirean, Sirean into solar type and those into red stars with banded spectra -although certain newly-found groups of stars, such as the Wolf-Rayet stars, did not automatically fit into it. The observation that the helium stars and gaseous nebulae have the same distribution about the galactic piano added support to the view that the white stars represented the youngest stage in the sequence. Also the inclusion in this class of the Algol-type variables, the prototype of which had been found by Vogel in 1888 to be a binary system, supported the general belief. Vogel had concluded that Algol was a binary system from a study of its radial velocity, obtained from measures of the shift in spectral lines throughout its cycle. The eclipsing nature of Algol was used by Picketing in 1880 to calculate its diameter, another important parameter in calibrating the evolutionary series. Most of the observations strengthened the belief in the general view of evolution. Disagreement was aimed rather at the causes of the changes in the spectral lines: was temperature the main factor involved, or density ? The answer was not forthcoming until the great advances in atomic theory in the early twentieth century. The Danish physicist Niels Bohr published his model for the structure of the atom in 1913. M.N.Saha used this model in 1920—1 to explain the appearance of the spectrum in terms of the excitation and ionization in stellar atmospheres, which could be calculated from the pressure and temperature of the atmosphere.

The extensive catalogues published during the second half of the nineteenth century provided virtually unlimited amounts of stellar data for the astronomers of the early twentieth century. The spectral classification developed at Harvard by E.C.Pickering and Antonia C.Maury was so detailed and extensive that all other classification schemes were soon swept into oblivion. In the first ‘Draper catalogue’, published in 1890, they proposed a classification scheme of stellar spectra including sixteen classes denoted according to the letters A-Q. The massive project was made possible by the introduction of the objective prism by Pickering in 1885 and funded as a memorial to Henry Draper, an early pioneer in astronomical photography. The actual classification scheme was some¬what modified with the publication of the second volume, including southern stars, in 1897 with certain groups being omitted or combined with other groups. At this time Maury added a second parameter (‘a’, ‘b’ or c) to classify the spectra, a parameter related to the sharpness of the line, since she noted that definite differences in sharpness did occur; this important factor was used by E.Hertz-sprung in 1905. The extensive work was continued by Annie J. Cannon with the resultant classification of over 225 000 stars in the catalogues published between 1918 and 1924 into the sequence of decreasing temperature accepted now 0, B, A, F, G, K, M, R, N, S. At the beginning of the twentieth century, the physical reasons behind such a temperature sequence still evaded astrophysicists. The catalogue has been hailed by leading astrophysicists as the greatest single work in the field of stellar spectroscopy.

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