THE COMPOSITION OF STARSSC
IEN
CE
PH
OT
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IBR
AR
Y
NEED TO KNOW
The spectrum of different stars are shown as horizontal strips:
dark ‘absorption’ lines betray the presence of various elementsUnderstand the
science of
spectroscopyFLAME TEST
The flame test is a simple way
to determine the identity of an
unknown substance. A clean
wire loop is dipped in the
substance (a compound, such
a s sodium chloride), then held
in the flame of a Bunsen burner.
The heat of the flame excites
the atoms (strictly speaking,
they’re ions), causing them to
emit visible light with a
characteristic colour: yellow
in the case of sodium.ION
An atom (sometimes a
molecule) that has lost one or
more of its electrons is
positively charged and called
an ion. The spectra of ions are
correspondingly different (in a
way that can be calculated)
from those of the ‘parent’
atoms. It is also possible for an
atom to gain an elec tron and
have an overall negative charge.STELLAR
SPECTROSCOPY
This is the study of the spectra
of s t arlight. In a hot ga s ,
collisions between fast-moving
atoms raise electrons to excited
s t ates. They then drop down,
producing emission lines. In a
cool gas, the electrons absorb
background light and are raised
to excited states. Spectra of
stars reveal which atoms are
involved and therefore what
stars are made of.sent a draft of her thesis to Henry
Norris Russell at Princeton for a
second opinion, he replied that the
result was “clearly impossible”.
On Shapley’s advice, Payne added a
sentence to her thesis saying that “the
enormous abundance derived for these
elements [hydrogen and helium] in the
stellar atmospheres is almost certainly
not real”. But with the thesis accepted
and her doctorate awarded, she wrote
a book called Stellar Atmospheres.Second opinion
The book was enough to persuade
astronomers that Payne’s results were
almost certainly accurate. This change
of mind was aided by the independent
confirmation of Payne’s results by her
fellow astrophysicists.
In 1928, the German astronomer
Albrecht Unsöld carried out a detailed
spectroscopic analysis of the lightfrom the Sun. He found that the
strength of the hydrogen lines implied
that there are roughly a million
hydrogen atoms in the Sun for every
atom of anything else. The following
year, the Irish astronomer William
McCrea confirmed these results using
a different spectroscopic technique.
Although many details remained
to be uncovered, by t he end of t he
1920s astronomers knew what the
philosopher Auguste Comte had
declared would forever remain beyond
our comprehension – that the Sun
and stars alike are mostly made of
hydrogen and helium, with traces of
other elements in proportions that can
be measured using spectroscopy.“Most of the matter in the Universe
was in the form of the two lightest
elements – hydrogen and helium”
by DR JOHN GRIBBIN
Dr Gribbin is a science writer and Visiting Fellow
in astronomy at the University of Sussex.