84
stellar spectra in relatively peripheral settings. The Jesuit Observatory of the
Collegio Romano, which was directed by Secchi from 1850 until his death in 1878,
became one of the internationally renowned centers of astronomical spectroscopy
and solar physics. It was in a poor state when he took it over, but Secchi relocated it
to a new facility on top of the Sant’ Ignazio Church (see Altamore et al., Chap. 1, in
this volume) and equipped it with high-quality instrumentation, some of which he
had invented or greatly improved, i.e., the heliospectroscope, the star spectroscope,
and telespectroscope (see Secchi 1870a, 1872; see also Johnson, Chap. 8, in this
volume).
His objective-prism method with a round 12°-prism (Fig. 5.1c) in front of his
telescope’s objective lens (a technique first used by Fraunhofer 50 years earlier and
by Secchi from 1855 on)
2
was later also adopted at Harvard, Potsdam, and other
observatories. Secchi’s superior instrumentation allowed him to reach the highest
dispersion in the stellar spectroscopy of his day.
At the Collegio Romano Observatory, Secchi examined about 4000 stars, including a few very bright stars (about 20 until 1863) with a direct-vision spectroscope
(Fig. 5.1a), most of the others with the objective prism (Fig. 5.1c) attached to his
equatorially mounted Merz refractor. With an aperture of 24 cm and a focal length
of 435 cm, it was an excellent instrument for its time.
3
After recording several hundred stellar spectrograms by hand, Secchi realized that stars come in a limited number of distinct types, which could be distinguished by means of their different
spectral patterns.
4
He thus developed the first stellar classification system. It initially
distinguished three, later four “Secchi classes” (published between 1863 and 1868).
More than half of these stars were bluish or white (like Sirius) and displayed little
besides the hydrogen lines; less than half were yellow (like the Sun), with numerous
fine dark lines like the Fraunhofer solar spectrum; while the small remainder constituted orange-to-red and dark red stars, with spectra incorporating both bright and
dark lines (Figs. 5.2 and 5.3).
5
Although the Secchi system was later superseded by the Harvard system (developed by Edward Pickering, Williamina Fleming, Antonia Maury, and Annie Jump
2 See McCarthy 1994: 231f. on Secchi’s acknowledgment of Fraunhofer’s priority and on the much
smaller angle of Secchi’s prism of 12° as opposed to Fraunhofer’s earlier 60° and 38° prisms.
3 On instruments by the Merz company, the successor to Fraunhofer, cf. Kost 2015, Chinnici 2017:
esp. 39–68 on the instrumentation at the Collegio Romano, on Secchi’s improvements and innovations. In Secchi 1870a and Secchi 1872: chapters II and VI, he himself explains the various instruments in detail.
4 This idea had been originally formulated in 1860 by Donati (see Donati 1862).
5 On the distinction between these four types of stars forming the basis of the Secchi classification
of stellar spectra (see table below), see Secchi 1863 about the first two types, Secchi 1866a,b for
the third type, and Secchi 1868a for the fourth. For summaries, see Secchi 1870b, Vogel and
Wilsing 1899: 258–329, Brück 1979: 12, Houziaux 1975, Hearnshaw 1986: 57–66, Hentschel
2002a: 347ff., and Cenadelli 2005 on the various stages of Secchi’s classification. See also Secchi’s
high-resolution drawing of the spectrum of α Orionis, dated February 9, 1866; it covers the full
range of the visible spectrum in a detailed pencil drawing of about 50 cm length, sent to Huggins
and preserved as a loose sheet in Huggins’s notebook 2 (Wellesley College Special Coll).
K. Hentschel
stellar spectra in relatively peripheral settings. The Jesuit Observatory of the
Collegio Romano, which was directed by Secchi from 1850 until his death in 1878,
became one of the internationally renowned centers of astronomical spectroscopy
and solar physics. It was in a poor state when he took it over, but Secchi relocated it
to a new facility on top of the Sant’ Ignazio Church (see Altamore et al., Chap. 1, in
this volume) and equipped it with high-quality instrumentation, some of which he
had invented or greatly improved, i.e., the heliospectroscope, the star spectroscope,
and telespectroscope (see Secchi 1870a, 1872; see also Johnson, Chap. 8, in this
volume).
His objective-prism method with a round 12°-prism (Fig. 5.1c) in front of his
telescope’s objective lens (a technique first used by Fraunhofer 50 years earlier and
by Secchi from 1855 on)
2
was later also adopted at Harvard, Potsdam, and other
observatories. Secchi’s superior instrumentation allowed him to reach the highest
dispersion in the stellar spectroscopy of his day.
At the Collegio Romano Observatory, Secchi examined about 4000 stars, including a few very bright stars (about 20 until 1863) with a direct-vision spectroscope
(Fig. 5.1a), most of the others with the objective prism (Fig. 5.1c) attached to his
equatorially mounted Merz refractor. With an aperture of 24 cm and a focal length
of 435 cm, it was an excellent instrument for its time.
3
After recording several hundred stellar spectrograms by hand, Secchi realized that stars come in a limited number of distinct types, which could be distinguished by means of their different
spectral patterns.
4
He thus developed the first stellar classification system. It initially
distinguished three, later four “Secchi classes” (published between 1863 and 1868).
More than half of these stars were bluish or white (like Sirius) and displayed little
besides the hydrogen lines; less than half were yellow (like the Sun), with numerous
fine dark lines like the Fraunhofer solar spectrum; while the small remainder constituted orange-to-red and dark red stars, with spectra incorporating both bright and
dark lines (Figs. 5.2 and 5.3).
5
Although the Secchi system was later superseded by the Harvard system (developed by Edward Pickering, Williamina Fleming, Antonia Maury, and Annie Jump
2 See McCarthy 1994: 231f. on Secchi’s acknowledgment of Fraunhofer’s priority and on the much
smaller angle of Secchi’s prism of 12° as opposed to Fraunhofer’s earlier 60° and 38° prisms.
3 On instruments by the Merz company, the successor to Fraunhofer, cf. Kost 2015, Chinnici 2017:
esp. 39–68 on the instrumentation at the Collegio Romano, on Secchi’s improvements and innovations. In Secchi 1870a and Secchi 1872: chapters II and VI, he himself explains the various instruments in detail.
4 This idea had been originally formulated in 1860 by Donati (see Donati 1862).
5 On the distinction between these four types of stars forming the basis of the Secchi classification
of stellar spectra (see table below), see Secchi 1863 about the first two types, Secchi 1866a,b for
the third type, and Secchi 1868a for the fourth. For summaries, see Secchi 1870b, Vogel and
Wilsing 1899: 258–329, Brück 1979: 12, Houziaux 1975, Hearnshaw 1986: 57–66, Hentschel
2002a: 347ff., and Cenadelli 2005 on the various stages of Secchi’s classification. See also Secchi’s
high-resolution drawing of the spectrum of α Orionis, dated February 9, 1866; it covers the full
range of the visible spectrum in a detailed pencil drawing of about 50 cm length, sent to Huggins
and preserved as a loose sheet in Huggins’s notebook 2 (Wellesley College Special Coll).
K. Hentschel