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| The Andromeda Galaxy is moving towards us with relative speed 225 Km/s (Credit: NASA/JPL-Caltech) |
The speed of galaxy is measured by observing the light
spectrum coming from the distant galaxy. The properties of spectrum change due
to the motion of galaxy with respect to the Earth. In other words you can say
that the spectrum contains the information about the motion of galaxy. The
Doppler Effect is used to determine the speed of the galaxy.
Doppler Effect
You must have observed that the sound of Horn of rail engine
is different when it is approaching as compared to the situation when it is
moving away. When the engine is approaching you, the sound is shriller
(sharper) as compared to the engine moving away because the frequency of sound increases
as the source moves towards the observer and decreases as it moves away from
the observer. This is called Doppler
Effect.
Red Shift
The Doppler Effect is also observed for light. When the source of light moves away from the observer the wavelength increases and the frequency decreases. Since the red light has largest wavelength in visible spectrum, the increase in wavelength is interpreted as the shift of wavelength towards red end of the spectrum. That is why it is also called red shift. The galaxies moving away from the Earth will appear redder than their actual colour.
Blue Shift
The opposite happens for the galaxy moving towards the
observer, the wavelength decreases and the frequency increases. The decrease in
wavelength is interpreted as the shift of wavelength towards the blue end of
visible spectrum. That is why it is also called blue shift. The galaxies moving towards the Earth will appear bluer
than their actual colour.
Relation between change in wavelength and speed of galaxy
The speed of galaxy is related to the change in wavelength
of light by following relation:
(λ’- λ)/λ=v/c
Here, λ = rest wavelength, λ’ = observed wavelength, c =
velocity of light, v = velocity of galaxy.
(λ’- λ)/λ is also called rest shift. Then,
velocity of galaxy = red
shift x velocity of light
Observation
The light from the distant galaxy is observed through the
spectrometer which spreads the light into spectrum. The lines of high intensity
can be easily observed which are usually due to hydrogen, oxygen and other elements
found in the galaxies. The red line of hydrogen (H alpha) can be easily
identified due to high intensity then the wavelength of this red line is
calculated which gives us λ’ for the red line of hydrogen.
The wavelength of red line of hydrogen in rest frame in lab
is also calculated which gives us λ. Now the red shift and velocity of galaxy
can be calculated from the above formula.
Example
Line = H alpha
Rest wavelength, λ = 6562.8 A
Observed wavelength, λ’ = 8100 A
Red shift = 0.234
Velocity of galaxy = red shift x velocity of light
= 7.03 x 104 km/s.