116
See also: The sun’s spectrum 112 ■ Nuclear fusion within stars 166–67 ■
The primeval atom 196–97I
n August 1868, the French
astronomer Pierre Jules César
Janssen traveled to India to
observe a solar eclipse. The eclipse
covered the sun’s bright disk,
leaving only a narrow ring of light.
This was the chromosphere, the
middle of three layers in the sun’s
atmosphere, which was normally
hidden by the glare. Janssen
found that the spectrum of the
chromosphere’s light contained
numerous bright emission lines.
Using discoveries made by Gustav
Kirchhoff, Janssen was able to
confirm that the chromosphere
was a layer of gas. He also noticed a
previously unseen yellow emission
line in the sun’s spectrum. He
assumed this unknown light was
produced by sodium, helping to
give the sun its yellow hue.
In October that year, English
astronomer Joseph Norman Lockyer
developed a spectroscope for
observing the chromosphere directly.
He also detected its curious light
and also assumed it was produced
by sodium, but after consulting
the chemist Edward Frankland,he changed his mind—the light was
not from sodium but from a hitherto
unknown element, which he named
helium, after helios, the Greek word
for the sun. For some years, it was
thought that helium only existed
on the sun, but in 1895, Scottish
chemist William Ramsay succeeded
in isolating a sample from a
radioactive uranium mineral. ■HE SUN’S YELLOW T
ROMINENCE P
DIFFERS FROM ANY
TERRESTRIAL FLAME
THE SUN’S EMISSIONS
IN CONTEXT
KEY ASTRONOMERS
Jules Janssen (1824 –1907)
Joseph Norman Lockyer
(1836–1920)BEFORE
1863 Gustav Kirchhoff develops
spectroscopy, showing how
light can be used to identify
hot substances.1864 William and Margaret
Huggins find that the spectra
of nebulae contain different
emission lines, showing that
they are largely clouds of gas.AFTER
1920 Arthur Eddington states
that stars are fueled by the
fusion of hydrogen into helium.1925 Cecilia Payne-Gaposchin
shows that stars are largely
made from the elements
hydrogen and helium.
1946 US cosmologist Ralph
Alpher calculates that most
of the universe’s helium was
formed in the first few minutes
after the Big Bang.A total eclipse of the sun reveals
the chromosphere. This image of an
eclipse was captured in 1919 by British
astronomer Arthur Eddington.