The Doppler Effect
The Doppler Effect and Cosmology
The study of the structure and development of the Universe as a whole is called Cosmology.
The Doppler effect is a change in frequency, or wavelength
of a wave for an observer moving relative to its source. The
same phenomenon occurs with all other types of waves,
including electromagnetic radiation.
Depending on the motion of the object with respect to the
observer, the frequency – and hence colour in the case of
light – is affected.
The colour of an approaching light source is a shifted to the
blue (shorter wavelength) than it would otherwise be, and
the colour of one that is moving away is shifted to the red (longer wavelength), as shown left.
The lines in a star’s absorption spectrum are shifted when compared to the same lines as measured in a laboratory.
This is due to the motion of the star relative to the Earth. The size of the wavelength shift, ∆ 𝜆, depends on the
relative velocity of the star and the observer. The relationship is given by the
Doppler equation:
∆𝜆 𝜆𝑎𝑝𝑝 − 𝜆 𝑉
𝑧= = =−
𝜆 𝜆 𝑐
∆𝜆
The quantity 𝜆 is termed the Doppler shift and is given the symbol z.
Here 𝜆 is the true wavelength of the absorption line, 𝜆app is the apparent
wavelength of the observed absorption line on Earth, v is the relative velocity
of the star and the Earth, and c is the velocity of light. The relative velocity v
is taken to be the relative velocity of approach, so that v is positive when the
two objects are approaching (blue shift) one another and negative if they are
receding (red shift).
The Doppler equation can also be expressed in terms of the change in
∆𝑓 𝑉
frequency: 𝑓 = 𝑐
Doppler Shift and the Motion of Binary Stars
The Doppler effect can be used to determine the rotational velocity and the
distance between two stars in a binary star system.
Here, eclipsing binaries, are revealed by the Doppler shift of lines in their
spectrum. The absorption lines seen from Earth will be Doppler-shifted as
the stars rotate about their centre of mass, moving between longer and
shorter wavelengths in a periodic
motion.
There will be a blue-shift in the
star’s spectral lines when the star is moving towards the Earth, a red-shift
in the spectral lines when the star is moving away from the Earth, and no
spectral shift in the lines when the star is moving perpendicularly to the
line of sight.
Analysis of the spectral motion reveals a cyclic movement of a particular
absorption line, shifted one way and then the next with a constant period
It is possible to calculate the linear velocity along the line of sight and the
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period, hence the distance between the two stars using the mechanics of circular motion.
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