The Doppler Effect
The Doppler effect causes waves, such as sound and light to change
wavelength depending on the velocity of the observer in relation to the
velocity of the emitter of the wave.
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The Doppler effect is expressed with the relation:
Δλ/λ(rest)
= v/c
v is the radial velocity of the emitter when the observer perceives themselves to be at rest.
λ(rest) is the wavelength of light (or sound) that would be observed from the object if it were at rest relative to the observer.
Δλ is the difference between the observed wavelength and λ(rest).
c is the speed of light.
In stars with similar surface temperatures as the Sun, the spectral absorption of ionised calcium feature prominent H and K lines. These lines can be studied in an Earth-bound laboratory quite easily to determine the value of λ(rest). The K line has a λ(rest) of 393.37nm and the H line has a λ(rest) of 396.85nm.
Because these lines are easily observable in distant galaxies, a spectrometer can be used to determine the value for λ for both lines. Due to the Doppler Effect, the values for λ and λ(rest) for each line will differ by the same Δλ. Using the equation Δλ/λ(rest) = v/c, the radial velocity of the galaxy can be experimentally determined.
λ(rest) is the wavelength of light (or sound) that would be observed from the object if it were at rest relative to the observer.
Δλ is the difference between the observed wavelength and λ(rest).
c is the speed of light.
In stars with similar surface temperatures as the Sun, the spectral absorption of ionised calcium feature prominent H and K lines. These lines can be studied in an Earth-bound laboratory quite easily to determine the value of λ(rest). The K line has a λ(rest) of 393.37nm and the H line has a λ(rest) of 396.85nm.
Because these lines are easily observable in distant galaxies, a spectrometer can be used to determine the value for λ for both lines. Due to the Doppler Effect, the values for λ and λ(rest) for each line will differ by the same Δλ. Using the equation Δλ/λ(rest) = v/c, the radial velocity of the galaxy can be experimentally determined.
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