Prismatic spectra by Fr. Secchi

Angelo Secchi,
”Sugli spettri prismatici delle stelle fisse”
Atti della Società Italiana delle Scienze detta dei XL
Serie III, tomo I, parte I, 1867, pp. 67-104.
Tav. II
260 x 290 mm
Library of INAF-Palermo Astronomical Observatory

Between 1867 and 1869 Angelo Secchi printed two memoirs «On the Prismatic spectra of fixed stars». Both were presented to the Italian Society of XL and published in the Proceedings of the company; Secchi, however, did also publish the extracts of the acts separately. The second memory bears the subtitle “Second Memory” and specifies that it was presented to the Italian Society in November 1868, while the first was presented in 1867. The two memories constitute a single work, asSecchi specifies in the introduction to the second.
The first part of the first memory is dedicated to the detailed description of the direct vision spectrometers developed and used by Secchi for the observation of the stars and of the artificial sources of comparison, depicted in the Tav. I (Figure 1). The central part of the memory is the catalogue of the stars observed. Here the most innovative part is section VI, entitled ‘ General considerations for the intelligence of the catalogue ‘. In This section, Secchi introduces specifically for the first time a spectral classification of the stars, here depicted , and uses the term “spectral type”. He classifies the stars into 3 types. The first type is made by the hot stars (type A or B, according to the modern classification) and the prototype is Vega (α Lyrae). The second type consists of very few stars, twelve in all, on a total of 316 stars catalogued. The prototype star is α Herculis and the class mostly includes those that are now called super-red giants, although there are also red or yellow giants. The third type are stars with a sun-like spectrum; Secchi proposes as a prototype the solar spectrum reflected by Venus. The type 3 stars of Secchi are nowadays classified with types F, G or K. The panel shown here depicts the three spectral types (1 to 3), together with spectra of other stars in the catalogue (from 4 to 7).
In sections VIII to XI Secchi describes the spectra of planets, of nebulae (mostly molecular clouds and planetary nebulae, but also the spectrum of the Andromeda Nebula, The Great Spiral galaxy), of the comet of Tempel and of variable stars (beginning From Algol, are all eclipsing binary stars). One of the most important conclusions of this extensive memory is that very few types are enough to classify the spectra of all the stars.
The second memory presents the improvements applied to the instruments of observation, compared to those described in the first. The central part consists of a description of the spectral study of “coloured” stars (sic) contained in a catalogue published by Schjellerup on Astronomische Nachrichten. Following the study of these stars Secchi is forced to introduce a fourth spectral type, which prototype is the star n. 152 of the catalogue of Schjellerup, nowadays known as HD 110914, but also as “the superb”, name that has resumed the record made by Secchi in the table of this memory in which it lists the stars of the fourth type. These are cool stars in the luminous part of the asymptotic branch of the giants, characterized by an overabundance of carbon, following the blending that brought some of the products of the nucleosynthesis occurred in the center of the star to the surface of it. It is a relatively rapid evolutionary phase, so they are rare but very bright stars; Given the choice of Secchi to study the stars of Schjellerup, which are red, it is not surprising that he found out several ones and that due to the peculiarity of their spectrum has deemed necessary to introduce a fourth type.
In section IV Secchi considers the effect of the motion of the star on the spectrum. He believes that the original idea of Doppler, to measure the radial motion of the star thanks to the change in color, is “practically impossible”, and proposes instead to measure the shift of the absorption lines . In particular, he notes that, with the instruments available at the time, a displacement of 304 km/s of sodium lines would have been measurable.
It then describes the unsuccessful attempts to measure such shift by observing high-motion stars, for which a high radial velocity was also expected. Nowadays the measurement of radial velocities thanks to the Doppler displacement of the spectral lines has become an ordinary practice; the precisions were at about 1 m/s and allowed the discovery of the first extrasolar planet around a main-sequence star and many others.
The final part of the memory is dedicated to the description of the spectra of comets and planets.
It follows the catalogue of the Stars observed, with many drawings of the spectra. The final part of the publication is an addition to the first memory, in which the several measures carried on are described, a description of the instruments and an appendix on the spectrum of the solar protuberances.
An interesting and extended third memory, dated November 9, 1869, can be found in the proceedings of the Italian Society of Sciences, entitled “On the prismatic spectra of the celestial Bodies”. It describes the spectral researches of Secchi about Sun, the star on which, since that moment, he focused his studies.
Later, Secchi summed up and enlarged the two memoirs in a single memory, which resumed the title of the third and which was published, in 1872, in the Proceedings of the new Lyncean Academy.

Piercarlo Bonifacio, Observatoire de Paris