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2.2.5Space

Spectral Data for Stars

5 min read 5 parts Lesson 12 of 12

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Part 1 of 5What Star Light Reveals

2.2.5 Spectral Data for Stars

A single beam of starlight carries information about the star itself.

Composition
  • Dark absorption lines in a star's spectrum show which elements are in its outer layers.
Main elements
  • Most stars are made mainly of hydrogen and helium, with smaller amounts of heavier elements.
Colour and temperature
  • Hotter stars look blue-white while cooler stars look red, so colour gives a guide to temperature.
Motion
  • The positions of the lines show whether the star is moving towards or away from Earth.
Star colourRelative surface temperature
RedCoolest
YellowIntermediate (like the Sun)
WhiteHotter
Blue-whiteHottest

Part 2 of 5Red-Shift and Blue-Shift

The Doppler effect changes the observed wavelength of light from moving sources.

Red-Shift and Blue-Shift
Receding source
  • If a star moves away, its light is stretched and the lines shift towards the red end of the spectrum.
Approaching source
  • If a star moves towards us, the lines shift towards the blue end.
Size of the shift
  • A larger shift means a greater speed, which is why distant galaxies show big red-shifts.
Using a reference
  • The shift is found by comparing observed line positions with the same lines measured in a laboratory.

Part 3 of 5Analysing a Spectrum

A systematic approach turns a diagram of lines into conclusions.

1
Read the pattern of lines in the star spectrum and note the position of each one.
2
Compare these lines with the reference spectra of elements printed in the question.
3
List every element whose complete set of lines can be found in the star spectrum.
4
Check whether the whole pattern has moved towards the red or blue side of the reference.
5
State the conclusion clearly, naming the elements and the direction of motion.

Part 4 of 5Worked Example

Question

  • Element A has lines at positions 1 and 3, element B at 2 and 4, and element C at 3 and 5.
  • A star spectrum shows lines at 1, 2, 3 and 4.
  • Which elements are present?

  1. Check Alines 1 and 3 both appear in the star, so A is present.
  2. Check Blines 2 and 4 both appear, so B is present.
  3. Check Cit needs line 5, which is missing, so C is not shown to be present.

Answer

  • Elements A and B are present;
    • there is no evidence for C.

Part 5 of 5Exam Checklist & Tips

2.2.5 – Key Points for the Exam

  • Absorption spectrum A continuous spectrum crossed by dark lines caused by gas absorbing light.
  • Red-shift Movement of spectral lines towards longer wavelengths when a source moves away.
  • Blue-shift Movement of spectral lines towards shorter wavelengths when a source moves closer.
  • Reference spectrum The known line pattern of an element, used for comparison.

Common Mistakes

  • Naming an element when only some of its lines appear in the star spectrum.
  • Mixing up red-shift with a star simply being red in colour.
  • Stating that red-shift means a star is approaching; it means it is moving away.
  • Forgetting to compare against the reference position of the lines before judging a shift.
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