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Cosmology: 3. 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 light?

how to analyse light fronm stars using diffraction grating prism

Reminders 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.

  • How do we gather evidence for an expanding universe?
  • If 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 an 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 galaxies. 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!).
  • emission spectra hydrogen helium neon sodium mercury line spectrum to identify elements in starsSpectra of elements
  • 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 right 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).

visible light absorption spectra for hydrogen line spectrum

  • Focus on the absorption spectrum of hydrogen atoms.

  • In the above diagram, the prominent vertical black lines are where the light from hot hydrogen atoms has been absorbed by the gases of the star.

    • They are numbered 1, 2, 3, etc. in the visible part of the emission spectrum of hydrogen.

  • 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 by H 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 spectrum.

  • 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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