Drawings of star clusters, nebulae, planets, sunspots

Angelo Secchi
Memorie dell’Osservatorio del Collegio Romano, anni 1852-55
Roma, Tipografia delle Belle Arti, 1856
Tav. V
300 x 380 mm
Biblioteca della Specola Vaticana, Albano Laziale (Roma)

Starting from 1852, Secchi pointed his excellent Merz telescope towards weak and distant objects. Among his early astrometric studies, in addition to the catalogue of double stars, he also dealt with stellar clusters (calling them “groups”) and nebulae. As for stellar clusters, Secchi was convinced that it was not a simple perspective effect but that there was a physical link between the stars of the same cluster, a bond of which it appeared long and complex-but not impossible-to find the law. About the nebulae, Secchi was convinced of the dual nature of these objects, which in some cases appeared as clusters of unresolved stars, but also as objects formed by other material, not better specified: spectroscopy showed then the gaseous nature of the so-called planetary nebulae, name deriving from their extended brightness, as of planet.
The table shown here, dated 1856, sums up well these initial interests of Secchi. On the left, we find the drawings of some stellar clusters: 1. Sagittarius (Caustics Cloud); 2. Sagittarius (now known as NGC 6603); 3. Group of Antinous (now known as NGC 6709, in the constellation Aquila, which replaced that of Antinous, now no longer existent); 5. Globular cluster of Hercules (otherwise known as M13, according to Charles Messier’s catalogue). Figure 4 shows a drawings of the nebula θ’ Orion (now known as the open cluster of the Trapezium, central part of the Orion Nebula); It was often observed by Secchi, who carried out an accurate study of it between 1864 and 1866, analyzing also its spectrum. In addition, figures 6 and 7 reproduce Jupiter and its third satellite, Ganymede, while figures 8, 10 and 11 illustrate Saturn, details of its ring and Mars. Finally, figure 9 shows the large sunspot observed by Secchi in May 1855, whose study confirmed his ideas about the nature and formation of sunspots.

Ileana Chinnici, INAF-Osservatorio Astronomico di Palermo

A. Secchi, Memorie dell’Osservatorio del Collegio Romano, anni 1852-55 (Roma, Tipografia delle Belle Arti, 1856); A. Secchi, Sulla grande nebulosa di θ’ Orione (Firenze, Stamperia Reale, 1868); A. Secchi, Memorie dell’Osservatorio del Collegio Romano, anno 1851- Appendice (Roma, Tipografia delle Belle Arti, 1852).