113
See also: The sun’s spectrum 112 ■ The sun’s emissions 116 ■
The star catalog 120–21 ■ The characteristics of stars 122–27A
ngelo Secchi was
one of the pioneers of
astrophysics, an arm
of science that focuses on the
properties of a star, rather than
merely its position in the sky.
He was the first to group stars
according to their spectra, or the
particular colors of light they emit.
A Jesuit priest as well as a
noted physicist, Secchi founded
a new observatory at the order’s
Collegio Romano in Rome. There he
became a pioneer of the technique
of spectroscopy, a new way to
measure and analyze starlight.
Gustav Kirchhoff had shown
that gaps in a stellar spectrum
were caused by the presence of
specific elements (see opposite).
Armed with this knowledge, Secchi
began to class stars according to
their spectra. At first he used three
classes: Class I were white or blue
stars that showed large amounts of
hydrogen in their spectra; Class II
were yellow stars, with metallic
spectral lines (for astronomers,
“metallic” refers to any element
heavier than helium); and Class IIIwere orange, with a complex array
of elements present. In 1868, Secchi
added Class IV for redder stars with
carbon present, and finally in 1877
came Class V for stars that showed
emission lines (not absorption lines,
as in the other four).
Secchi’s stellar classes were
later amended by other scientists,
and in 1880 became the foundation
of the Harvard System, which is
used to classify stars to this day. ■THE RISE OF ASTROPHYSICS
STARS CAN BE
GROUPED BY
THEIR SPECTRA
ANALYZING STARLIGHT
IN CONTEXT
KEY ASTRONOMER
Angelo Secchi (1818–1878)
BEFORE
1802 William Hyde Wollaston
notices that there are dark
gaps in the sun’s spectrum.
1814 German lensmaker
Joseph von Fraunhofer
measures the wavelengths
of these dark lines.
1860 Gustav Kirchhoff and
Robert Bunsen use a gas
burner to make systematic
recordings of the wavelengths
produced by burning elements.
AFTER
1868 English scientist
Norman Lockyer identifies
a new element, helium,
from emission lines in the
sun’s l ight.
1901 The Harvard System
for the classification of
stellar spectra, devised
by Williamina Fleming
and Annie Jump Cannon,
supersedes Secchi’s system.
Spectroscopy uses
a prism to refract
the light from a star,
splitting the light to
allow its constituent
wavelengths to be
measured with a high
degree of accuracy.PrismLightStarSpectrum