THE FUNDAMENTALS OF PHYSICS

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Rober t Bunsen and Gust av Kirchof f found pat terns

of bright lines in the spectra of heated elements

THE COMPOSITION OF

STARS

Unlocking the meaning of features in the spectrum of sunlight

enabled us to identify elements present in stars. Dr John Gribbin

explains how it also gave rise to a new science – astrophysics

T

he philosopher Auguste Comte
w rote, in 1835, t hat “t here is no
conceivable means by which we
shall one day determine the chemical
composition of the stars”. So much for
philosophy. By the time Comte died in
1857, astrophysicists were well on the
way to finding out what stars are made
of. Indeed, spectroscopy, the tool they
would use to do it, had already been
invented decades before Comte made
his pronouncement.
In 1802, the British scientist William
Hyde Wollaston was studying sunlight
by passing it through a slit to make a
narrow beam and then through a glass
prism to spread the beam into a solar
spectrum. By doing so, he noticed that
the colours were separated by dark
bands – two in the red part of the
spectrum, three in the green and two
in the blue-violet region.
Wollaston thought these were
simply gaps between the colours, but
his discovery triggered the interest of
the German Joseph von Fraunhofer,
who was able to produce much more
detailed spectra in the second decade
of the 19th century. Eventually, he
identified 574 separate lines. Today,
all the dark lines in the solar spectrum
(even more t ha n he counted) a re
known as Fraunhofer Lines. A clue
to their origin emerged in the same
decade that Comte died, the 1850s.

It started with the work of Robert

Bunsen and Gustav Kirchhoff in

Ger ma ny. This is t he sa me Bunsen

whose name is known to everyone

who takes chemistry at school, thanks

to the famous burner. Early in the

1850s, t he city of Heidelberg had pipes

installed. These would distribute

inflammable gas derived from coal to

households and businesses – and to

the scientific laboratories of the

university. It was the inspiration for

Bunsen’s work with the burner that

now bears his name. The burner

combines oxygen and inflammable gas

in a controlled way, producing a clear

flame. It’s an ideal tool for a chemical

test in which substances are identified

by t he colou r t hey give to a f la me.

All in the detail

Bunsen originally used coloured

filters to calibrate these tests, but

Kirchhoff pointed out that it would

be possible to ma ke a more detailed

analysis. Together, they built an

apparatus that included a narrow

slit for the light to pass through,

a collimator to narrow the beam

and a prism to spread the light into a

rainbow. Finally, an eyepiece, like that

of a microscope, allowed them to view

the spectrum. This was the first time

all these components had been

assembled into a single instrument – a

spectroscope – although Fraunhofer

had previously used a prism and

eyepiece combination in his work.

Bunsen and Kirchhoff knew that

when different substances were put

in the clear flame of a burner, they

burned with different colours. A trace

of sodium, for example, turns the

flame yellow, while copper colours the

flame green/blue. When they analysed

the light from these flames using 5