12
subsequent freehand drawing. The consolidation of the photographic technique
applied to spectroscopy allowed the astronomer Edward Charles Pickering
(1846–1919) to begin an accurate classification project; the contribution of Draper’s
widow, who decided to fund the project in the memory of her husband, was also
decisive. Pickering published his first catalogue in 1891 (Draper Catalogue of
Stellar Spectra) based on the observation of 10,351 stars. The work continued in the
following years, thanks to the efforts of Pickering’s many collaborators and in particular Annie Jump Cannon (1863–1941). It was Cannon who put the letterclassification of stars into the modern sequence corresponding with surface
temperature: OBAFGKM. This immense work, mostly by women “computers,” as
they were called, resulted in the Henry Draper Catalogue containing the magnitude,
position, and classification of 225,300 stars, published in a series of volumes from
1918 to 1924. Even today, spectral classification remains a primary tool for describing a star that is in a vast set of stars and for identifying stars with a peculiar
composition.
The astronomers Ejnar Hertzsprung (1873–1967) and Henry Norris Russell
(1877–1957) had the happy notion of putting temperature information, from spectral class or from “color,” into a graph plotting color against absolute brightness of
the various stars (Fig. 1.6). The H-R diagram is a fundamental step toward understanding stellar evolution. As stars progress in their lifetime, instead of falling down
from hot white to warm red on the “Main Sequence,” as first thought, their changing
temperatures and brightness trace a complex path in the H-R diagram. This starts at
a point on the Main Sequence that depends on a star’s initial mass, and as the star
ages, its position moves upwards among the giants toward the supergiants, in accordance with the nature and region of the nuclear “burning” in its interior.
Because more massive stars start higher (hotter) on the Main Sequence and go
through their changes in temperature and brightness faster than those of lower mass,
Fig. 1.5 (I) Absorption spectrum characteristic of Secchi’s Type I stars. (Secchi 1877b). (II)
Absorption spectrum characteristic of Secchi’s Type II stars. (Secchi 1877b). (III) Absorption
spectrum characteristic of Secchi’s Type III stars. (Secchi 1877b). (IV) Absorption spectrum characteristic of Secchi’s Type IV stars. (Secchi 1877b). (V) Emission spectrum characteristic of
Secchi’s Type V stars. (Secchi 1877b)
A. Altamore et al.
subsequent freehand drawing. The consolidation of the photographic technique
applied to spectroscopy allowed the astronomer Edward Charles Pickering
(1846–1919) to begin an accurate classification project; the contribution of Draper’s
widow, who decided to fund the project in the memory of her husband, was also
decisive. Pickering published his first catalogue in 1891 (Draper Catalogue of
Stellar Spectra) based on the observation of 10,351 stars. The work continued in the
following years, thanks to the efforts of Pickering’s many collaborators and in particular Annie Jump Cannon (1863–1941). It was Cannon who put the letterclassification of stars into the modern sequence corresponding with surface
temperature: OBAFGKM. This immense work, mostly by women “computers,” as
they were called, resulted in the Henry Draper Catalogue containing the magnitude,
position, and classification of 225,300 stars, published in a series of volumes from
1918 to 1924. Even today, spectral classification remains a primary tool for describing a star that is in a vast set of stars and for identifying stars with a peculiar
composition.
The astronomers Ejnar Hertzsprung (1873–1967) and Henry Norris Russell
(1877–1957) had the happy notion of putting temperature information, from spectral class or from “color,” into a graph plotting color against absolute brightness of
the various stars (Fig. 1.6). The H-R diagram is a fundamental step toward understanding stellar evolution. As stars progress in their lifetime, instead of falling down
from hot white to warm red on the “Main Sequence,” as first thought, their changing
temperatures and brightness trace a complex path in the H-R diagram. This starts at
a point on the Main Sequence that depends on a star’s initial mass, and as the star
ages, its position moves upwards among the giants toward the supergiants, in accordance with the nature and region of the nuclear “burning” in its interior.
Because more massive stars start higher (hotter) on the Main Sequence and go
through their changes in temperature and brightness faster than those of lower mass,
Fig. 1.5 (I) Absorption spectrum characteristic of Secchi’s Type I stars. (Secchi 1877b). (II)
Absorption spectrum characteristic of Secchi’s Type II stars. (Secchi 1877b). (III) Absorption
spectrum characteristic of Secchi’s Type III stars. (Secchi 1877b). (IV) Absorption spectrum characteristic of Secchi’s Type IV stars. (Secchi 1877b). (V) Emission spectrum characteristic of
Secchi’s Type V stars. (Secchi 1877b)
A. Altamore et al.