The Solar Spectrum
ATLAS OF THE UNIVERSE
I
f we could do no more than examine the bright photo-
sphere, and follow the changes in the spots, faculae and
granules, our knowledge of the Sun would remain slender
indeed. Luckily this is not the case, and we can turn to that
other great astronomical instrument, the spectroscope.
Just as a telescope collects light, so a spectroscope
splits it up. A beam of sunlight is made up of a mixture of
colours, and a glass prism will bend or refract the various
colours unequally; short wavelengths (blue and violet) are
refracted most, long wavelengths (orange and red) least.
The first experiments were made by Isaac Newton in
1666, but he never followed them up, perhaps because
the prisms he had to use were of poor quality. In 1802 the
English scientist W. H. Wollaston passed sunlight through
a prism, via a slit in an opaque screen, and obtained a
true solar spectrum, with red at one end through orange,
yellow, green, blue and violet. Wollaston saw that the
rainbow band was crossed by dark lines, but he mistakenly
thought that these lines merely marked the boundaries
between different colours. Twelve years later Josef
Fraunhofer made a much more detailed investigation, and
realized that the dark lines were permanent, keeping to the
same positions and with the same intensities; he mapped
324 of them, and even today they are still often referred to
as the Fraunhofer lines. In 1859, two German physicists,
Gustav Kirchhoff and Robert Bunsen, interpreted themThe solar spectrum
The photosphere
produces a rainbow or
continuous spectrum
from red at the long
wavelength end to violet
at the shortwave end (A).
The solar atmosphere
should produce an
emission spectrum (B),
but as light is radiated
from the surface,
gaseous elements in
the atmosphere absorb
specific wavelengths, so
the spectrum observed
on Earth has gaps (dark
lines, called Fraunhofer
lines) in it (C).▲The Swedish 1-m Solar
Telescope, on La Palma,
started operation in 2002.
It is the largest optical solar
telescope in Europe, and
the second largest in the
world. Its system of adaptive
optics means that it can see
details on the Sun’s surface
as small as 70 km (43 miles). The Sunimaged in the
light of hydrogen (H-alpha)
by Don Trombino.Element Number of atoms,
the number of hydrogen atoms
being taken as 1,000,000
Helium 63,000
Oxygen 690
Carbon 420
Nitrogen 87
Silicon 45
Magnesium 40
Neon 37
Iron 32
Sulphur 16
All others Below 5RATIO OF ELEMENTS IN THE SUNA
B
C
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