Identifying Elements Within Stars Using Spectral Lines (OCR A Level Physics)

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Identifying Elements Within Stars Using Spectral Lines

  • Photons produced by fusion reactions in a star’s core move towards the layers of gas in the outer atmosphere of the star
    • The photons have all frequencies of the electromagnetic spectrum - this is known as a continuous spectrum
    • Photons are absorbed by the gas atoms, which excite and re-emit other photons of various frequencies in random directions
  • The light from a star can be analysed using spectroscopy
    • The atmospheres of stars are not hot enough to produce an emission line spectrum
    • Therefore, stars are found to emit an absorption line spectrum

Hydrogen Absorption Spectra

An absorption line will appear in a spectrum if an absorbing material is placed between a source and the observer

  • An absorption line spectrum is the equivalent of an emission line spectrum but it is made of dark lines on top of a continuous spectrum
    • The dark lines represent the frequencies or wavelengths that are absorbed by a medium, such as a gas, when light passes through it
  • Each gas produces a unique pattern of spectral lines due to the specific transition between the element’s energy levels
    • The presence of spectral lines in a star’s absorption spectrum act as fingerprints
    • They can be used to determine the presence of a certain element within the star


Emission line spectra are unique to each element


  • The chemical composition of a star can be investigated even when extremely distant
    • If the element is present in the star, its characteristic pattern of spectral lines will appear as dark lines in the absorption line spectrum of the star
  • The Sun is predominantly made up of hydrogen and helium gas
    • The chemical composition of the Sun can be verified using the emission line spectra of the two gases compared with the absorption line spectrum of the Sun
  • For example, the hydrogen emission line spectrum includes lines at:
    • 2 nm, 486.3 nm and 656.5 nm
  • While helium spectrum includes lines at:
    • 7 nm and 587.7 nm
  • The same wavelengths can be seen as dark lines on top of the Sun’s continuous spectrum

Exam Tip

Given an absorption line spectrum for a specific star, you can be asked to identify a star of similar chemical composition. It is important to pay attention to the spacing between the lines to be able to correctly identify the most similar star to the given one.

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.