21 March – 21 June 2016 in Florence, Rome, Naples, Palermo, Padua

ANGELO SECCHI (1818-1878)

Le Soleil: exposé des principales découvertes modernes sur la structure de cet astre, son influence dans l’univers et ses relations avec les autres corps célestes. Paris: Gauthier-Villars, 1870.
xii, 422 p.,: ill., tav.; 230 mm

INAF-Arcetri Astrophysical Observatory

The Jesuit Father Angelo Secchi resumed and extended Donati’s studies using the 25 cm Merz refractor of the Collegio Romano and various types of spectroscopes, including the objective-prism (see Rm 07). Starting from 1862, Secchi began a systematic program of stellar spectroscopy that led him to analyse more than 4,000 stars, and formulate a classification scheme based on fundamental spectral classes called “types”. This scheme, continually revised and updated by Secchi until shortly before his death, was developed starting from the two types of 1863 (white and coloured) to the three types of 1867 (white-blue, yellow, and orange-red), to which Secchi added in 1869 the fourth type of dark-red stars (carbon stars), and finally, in 1877, a fifth type, that of γ Cas (now called Be-stars), characterized by emission rather than absorption lines in their spectra.
The first edition of the book Le Soleil was published in Paris in 1870 and contains an exposition of Secchi’s spectral classification (see also Pa. 06 for the second edition, revised and expanded). Plate II shows at the top the characteristic spectrum of Type-2 stars, like the Sun and Pollux, with their many absorption lines due to various chemical elements identified by Gustav Kirchhoff (see Pa. 01). Then follows the spectrum of Type-1 stars, like Sirius and Vega, dominated by the presence of hydrogen absorption lines, and that of Type 3 stars, like α Her and β Peg, in which is evident the presence of thick absorption bands. Plate III presents another spectrum of Type-3 stars (like α Ori and Antares), followed by the spectra of two Type-4 red stars dominated by the absorption bands of carbon compounds.

Bibl.: SECCHI 1870; HEARNSHAW 2014.