Le Stelle: Saggio di Astronomia Siderale, Milano: Fratelli Dumolard, 1877.

VIII, 425 p.: ill., tav.; 210 mm

INAF-Arcetri Astrophysical Observatory

This book, published in 1877 and dedicated to Giovanni Virginio Schiaparelli, is the second work where Secchi presents its stellar classification scheme. It is interesting to compare the spectra of the Plates included in this volume with those from Le Soleil (see Fi. 03). In Plate III (Different types of stellar spectra) the solar spectrum, typical of Type-2 stars, is now divided into photosphere and chromosphere: the photospheric spectrum shows the well-known Fraunhofer absorption lines, whereas the chromospheric spectrum presents four emission lines of hydrogen (later called Hα, Hβ, Hγ and Hδ of the Balmer series) and the yellow line of helium. Plate III then shows the spectrum of the star Sirius, characteristic of Type-1 stars, and that of the stars α Ori and α Her, representative of Type-3. Plate IV (Different types of celestial spectra) continues presenting the spectra of two Type-4 red stars. This Plate shows two additional spectra of non-stellar objects: that of Coggia’s comet, observed in 1874 and characterized by bright emission bands (see Fi. 04), and the spectrum of a nebula, made only by emission lines. Among these, Secchi identifies the red and the blue lines as emission lines of hydrogen (for comparison with the solar spectrum), whereas the two green lines, attributed in 1864 by the British astronomer William Huggins to an unknown element called nebulio, will be identified only in 1927 as emission lines of doubly ionized oxygen.

Bibl.: SECCHI 1877, HEARNSHAW 2014.