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

• 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

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