Why do we receive light from stars? How do we analyse this
light to deduce the star's chemical composition?
Doc Brown's Physics exam study revision notes: There are various sections to work through,
best studied in from 1 to 6.
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(3) Why do we receive light from stars?
and how do we analyse this
from a simple optics experiment with a prism illustrated above.
When white light from a luminous source is passed
through a triangular prism (or diffraction grating) the different
wavelengths (and frequencies) of the electromagnetic radiation are
dispersed because their angle of refraction differs.
Think of the source
of white light as a distant star or galaxy - the colour is often
variable from one start to another e.g. white to yellow to red.
However, with stars, although the light seems 'white'
certain frequencies are more strongly observed than others. This is
illustrated by the emission spectral lines (specific frequencies)
observed for elements, each of which has its own characteristic pattern,
called the element's spectrum.
These 'chemical signatures' are illustrated in the
element spectra diagram below.
Each emission line spectra is unique for each element
(a sort of 'elemental' chemical signature) and is observed as a unique
different pattern of lines i.e. a 'spectral fingerprint'
This is used to
identify any element in the periodic table .e.g. the diagram on the above
shows some of the visible emission line spectra for the elements
hydrogen, helium, neon, sodium and mercury.
As well as emission spectra you can also observe an absorption
spectrum - this provides better evidence for the expanding universe
theory than emission spectra.
Absorption spectra are formed because the photon emissions from atoms
are absorbed by other atoms and instead of a coloured spectral line you
see a 'black' line, where that frequency of light is absent.
The spectral lines for hydrogen are particularly useful in
cosmology (See below and Parts 4 and 5).
do we gather evidence for an expanding universe?
the atoms of an element are heated to a very high temperature e.g. in a
star they emit
light of a specific set of frequencies (or wavelengths), called the
emission spectrum of an element.
These are all
due to electronic changes in the atoms, the electrons are excited at
high temperatures and
then lose energy by emitting energy as photons of light.
- These emitted
frequencies can be analysed with a diffraction grating or glass prism
and recorded on a photographic plate or digital camera. This is an example
of an instrumental chemical analysis called spectroscopy and is performed using an instrument
called an optical spectrometer.
Some schools may have a simple
mini version of a spectrometer, called a spectroscope, for you to look through, to give you
idea of what spectrum looks like eg looking at flame colours by heating
metals salts in a roaring bunsen flame.
- This type of optical spectroscopy producing emission spectra or
absorption spectra has enabled scientists to discover new
elements in the past and today identify elements in distant stars and
The alkali metals caesium (cesium) and rubidium were discovered by
observation of their line spectrum and helium identified from spectral
observation of our Sun (our nearest star!).
absorption spectrum of hydrogen atoms.
In the above diagram, the prominent vertical black lines are where the light from
hydrogen atoms has been absorbed by the gases of the star.
As we have seen, stars are so hot that the atoms
of hydrogen are in a gaseous state and due to electronic changes in
these hot H atoms, but, certain specific frequencies of visible light emitted
can be reabsorbed
by atoms of the same element.
This means certain hydrogen frequencies will be
'missing' and not be observed at all as a coloured line, but as a black line
with the visible rainbow spectrum in the background.
Therefore, when you examine the
visible light from distant stars and atoms such as hydrogen, you get black lines where that particular
frequency has been absorbed by other atoms i.e. these specific visible light
frequencies are missing.
The resulting 'picture',
obtained by using an instrument called a spectrometer, is called the
absorption spectrum (e.g. of hydrogen) based on the visible region of the electromagnetic
The spectra pattern is just like the emission spectrum line pattern because the
frequencies involved are identical, BUT no colour!
In the diagram, I've tried to
illustrate the idea using the spectral lines of the element hydrogen.
Hydrogen is the most abundant
element in stars, but all the other elements absorb visible light waves, so
the real absorption spectrum is much more complicated, but my diagram will
do here to teach you about the 'red shift' idea described in Part 4!
In the hydrogen spectrum diagram above, the first two
lines are red and green with lots of others in the blue-indigo-violet
region of the visible spectrum.
This is the pattern you observe when examining hydrogen
gas on Earth in the laboratory or the hydrogen in the Sun.
You can analyse the spectral data for
helium, its more complex, BUT, shows the same red shift pattern i.e.
characteristic absorption spectrum lines are shifted to lower
frequencies, longer wavelengths.
We will now combine the ideas from
parts 2. and 3. to
explain the 'red-shift' and its significance of our understanding of
universe - its origin and age in Parts (4) and (5)
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Keywords, phrases and learning objectives for
Know that you can analyse the spectra of light
emitted by stars using a spectrometer.
Know why do we get light from stars from very
electronically excited hot atoms.
Know how do we analyse this emitted light to produce
an emission spectra or absorption spectra.
Know that electronic changes in atoms cause emission of
photons to give characteristic emission or absorption spectrum lines from which an
element can be identified - its spectral signature.
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