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The Doppler Shift
The change of pitch because of movement is called the Doppler Shift
(named for Christian Andreas
Doppler). Sounds from objects moving towards you sound higher pitched than
when stationary and sound lower pitched when moving away. If you know what
the train horn sounds like standing still, you can figure out if a train in
moving towards or away from you and how fast just by measuring the frequency
(pitch) of the sound you hear! Cool! Of course, to do this you MUST
know the exact tone of the train horn "at rest".
standing near railroad tracks when a train passes blowing its horn (it works
with ambulances and sirens too). As the train passes the horn
goes Eeeee-owwwww! It starts out high pitched and as it passes the
tone seems to go down. If the train was just parked there (or if you were
on the train) the tone on the horn would be halfway between the "Eeee" and the
The Doppler Shift works with light exactly the same as with sound!
Just as sound seems higher or lower in pitch as the object is moving
towards or away from you, light appears higher in
frequency (color) when the object is moving towards you, and lower in
fequency/color when moving away from you. The order of
colors/frequency of light that we can see is:
red - orange - yellow
- green - blue - violet
So a far away object moving quickly away from you would appear more reddish
than if it were standing still.
The same object moving quickly towards you would
appear more blueish than if it were standing still.
it weren't for those Fraunhofer lines we wouldn't be able to ever tell if a
star were moving towards or away from us because we wouldn't know the exact
"tone" (frequency) of the star's light "at rest". Since we can measure
Fraunhofer lines here on Earth, we do know the "at rest" frequency!
Determing Movement from the Doppler Shift
In a lab we can measure the 2 yellow lines of sodium (like our salt and
alcohol demonstration) and can see that they are at 5880 and 5900 Angstroms
(a measure of the wavelength of light but that's another lesson).
If we see sodium
lines in the spectra of a star but the lines are at 5890 and 5910,
like the lower pitch of the train whistle, these are a lower
wavelength of light! So we would know the star was moving away from us.
If we measured 5800 and 5820, the star would be moving toward us!
Sodium spectrum redshifted (6000 and 5980 A.) -- object is moving
away from us
Sodium spectrum (5900 and 5880 A.)
Sodium spectrum blueshifted (5800 and 5780 A.) -- object is moving
OK, so far we've learned that
that spectral lines ("Fraunhofer lines")
tell us what gases are in the star or light source.
We also know that we can learn something about how a star
moves from it's Doppler Shift.
Don't Panic! Don't worry about things like "Angstroms".
Here's the game: if a star is coming towards you, its
Fraunhofer lines will move toward the blue end of the spectrum
If a star is moving away from us, its Fraunhofer lines will
move towards the red end of the spectrum.
Remember our bright yellow sodium lines?
If we saw them on a star (or nebula) coming towards us, the
lines might be green instead of yellow. If the star or nebula was
traveling away from us, the lines might be orange or red instead of
Thus the bluer or redder the sodium lines, the faster the star or nebula
is moving toward or away from us!
Do remember that the
star (or nebula) isn't just emitting "light" (photons) that we can
see. It also emits photons that are of less energy -- below" red,
which we call infra-red and radio waves.
And, it also emits photons that are of greater energy,
past or above the violet we can see, hence
ultraviolet, x-rays, and maybe even gamma rays.
There are also
Fraunhofer lines in these wavelengths that we cannot see with our eyes.
If a star is moving towards us, its
light "blue shifts" and Fraunhofer lines that have been hiding down in the
infra-red are now visible in the red or orange.
In the same way, a star
moving away from us may have "red shifted" its light
and Fraunhofer lines from up in the
UV (ultraviolet) to make then now visible down in the blue. Simple!