SCIENCE&^SOCIETY,^ SCIENCE^ PHOTOLIBRARY,^GETTY^ X2 ,^ NASA5 Norman Lockyer. His greatest
achievement ca me on 20 October
1868 when he a nalysed light f rom
the outer layers of the Sun with a new
spectroscopic instrument. These
observations followed hot on the heels
of a spectroscopic study of the outer
layers of the Sun during an eclipse
visible from India on 18 August that
year. The observations were made by
the French astronomer Pierre Janssen.
With the Moon blocking out the bright
light from the surface of the Sun, he
could detect lines in the spectrum of
the material just above the surface. He
noticed bright lines in the spectrum
of this layer of the Sun’s atmosphere,
known as the chromosphere,
including a bright yellow line, close
to but distinct from the sodium lines.
Its wavelength was later measured as
587.49 nanometres.
On 20 October t hat sa me yea r,
unaware of Janssen’s work, Lockyer
used his new spectroscope to observe
the solar atmosphere and found the
same yellow line. Both Janssen’s and
Lockyer’s discoveries were presented
to the French Academy of Sciences on
26 October 1868. But it was Lockyer
who took things a step further by
claiming that the line must be
associated with a previously unknown
element, which he called helium, from
the Greek word for the Sun: helios.
This was a controversial claim. But
in 1895, the physicist William Ramsay
found that a previously unknown gas
released by uranium produced a bright
yellow line near to the sodium lines in
the spectrum. He initially called this
gas krypton. But when his colleague
William Crookes pointed out that the
line was in exactly the same place as
the one found in the solar spectrum
by Lockyer, Ramsay realised it was in
fact helium. In effect, spectroscopy
had predicted the discovery of helium
on Ea r t h, 27 yea rs in adva nce.Payne the pioneer
The next step was taken by Cecilia
Payne. Born in 1900, she won a
scholarship to Newnham College,
Cambridge in 1919, where she studied
botany, physics and chemistry, but
could not be awarded a degree(Cambridge did not award degrees to
women until 1948). So, in 1923, she left
for the United States. Just two years
later, she produced a brilliant thesis
and established that the Sun is made
mainly of hydrogen. But, in a sign
of the times, the idea was not fully
accepted until two male astronomers
came to the same conclusion.
By t he 1920s, physicists k new (as,
of course, Bunsen and Kirchhoff had
not) that atoms are composed of a tiny
central nucleus, with one or more
electrons in orbit around it. Dark lines
in a spectrum are produced when an
electron absorbs a specific wavelength
of light, moving to a higher energy
level within the atom. Bright lines are
produced when a n elect ron d rops
down from one energy level to another
and emits radiation (in the form, we
would now say, of a photon of light).
Payne measured the absorption lines
in stellar spectra and showed how the
temperature (in particular) and
pressu re in t he at mosphere of a sta r
affects the ionisation of the atoms
t here. Ionisation is when a n atom or
molecule gains or loses electric charge
(see ‘Need to K now’, opposite). The
spectra of stars differ from one another
not because they are made of different
things, but due to different amounts of
ionisation in their atmospheres.
Payne unravelled this complicated
pattern of hundreds of Fraunhofer
lines and worked out what proportion
of different elements in different
stages of ionisation had to be present
to account for the observations. She
calculated the proportions of 18
elements in the Sun and stars,
discovering they all had nearly the
same composition. But the big surprise
was that the Sun and stars are made
almost entirely of hydrogen and
helium. Everything else put together
made up only two per cent of t he
composition not only of our nearest
star, but of all stars. Most of the matter
in the Universe was in the form of
the two lightest elements – hydrogen
and helium.
Such a notion was almost
unbelievable in 1925, but Payne
believed her results were correct. Yet
when her super visor Ha rlow Shapley1814
1859
1802
William Hyde Wollaston
publishes his analysis
of astronomical spectra
in the Philosophical
Transactions of the
Royal Society. He was the
first person to notice the
presence of dark bands in the
Sun’s spectrum.1925
Cecilia Payne presents her PhD thesis
in which she provides measurements
implying that the Sun is overwhelmingly
composed of hydrogen.1928
Albrecht Unsöld also
suggests that hydrogen
really is the most common
stellar element. A year
later, William McCrea
(right), a research student
at Cambridge University,
confirms the result.Joseph von Fraunhofer begins an
investigation of the dark lines in the
solar spectrum, which become known
as Fraunhofer Lines. He accurately
measured the wavelengths of nearly
600 of these lines.Robert Bunsen (left)
and Gustav Kirchhoff
detect spectral lines
produced by strontium
and barium in a fire
raging at Mannheim,
16km away from their
lab in Heidelberg.1868
Pierre Janssen and Norman Lockyer
independently discover lines in the
spectrum of light from the Sun that
do not correspond to
those produced by
any known element.
Dubbed helium, the
element was not
found on Earth
until 1895.TIMELINE
THE FUNDAMENTALS OF PHYSICS