Transcript The Universe is Expanding – Galactic Redshift

The Universe is Expanding – Galactic Redshift
• Recall from waves, that the frequency of a wave can
be distorted by relative motion – The Doppler Effect.
Light from distant galaxies
also exhibit the Doppler Effect
Recall the structure of the Atom
Recall the structure of the Atom
Recall the structure of the Atom
Recall the structure of the Atom
Absorption and Emission Spectra
62 52
42
n=32
n=6
n=5
n=4
n=3 (2nd excited state)
n=2 (1st excited state)
Larger Jump = More Energy = Bluer Wavelength
n=1 (Ground State)
26 25
24
n=23
n=6
n=5
n=4
n=3 (2nd excited state)
n=2 (1st excited state)
Photons of all other energies (wavelengths) are
ignored and pass on by unabsorbed.
n=1 (Ground State)
Hydrogen
Helium
Oxygen
Neon
Iron
The Suns Absorption Spectrum
The Suns Absorption Spectrum from 420 – 430 nm. (TOP)
The emission spectrum of Iron (Bottom)
Galactic Redshift
• When looking at the spectral lines of distant
galaxies, we see a shift in the spectral lines.
• example
R
O
Y
G
B
Earth Lab
Galaxy

I
V
The Universe is Expanding – Galactic Redshift
• Recall from waves, that the frequency of a wave can
be distorted by relative motion – The Doppler Effect.
Light from distant galaxies
also exhibit the Doppler Effect
Galactic Redshift
• Using the Doppler Effect – we can state
v
1
c where v  recessional velocity
'  
v
1
c
R
O
Y
G
B
Earth Lab
Galaxy

I
V
Teacher Note: The
derivation can been seen
on page 638 IB Book by
Paul Ruth
Galactic Redshift
• If v <<< c then …
v
1
 v
'
c
 
becomes approximately
 PSRT
v

c
1
c
R
O
Y
G
B
Earth Lab
Galaxy

I
V
• Example Problem 1
– The wavelength of a particular spectral line in the star
Megrez in the Big Dipper (Ursa Major, aka the Great Bear) is
486.115nm. Laboratory measurements demonstrate that the
normal wavelength is 486.113nm. What speed is this star
moving relative to the Earth?

v  ( )( c )

v  1, 234 m / s
R
O
Y
G
B
Earth Lab
Galaxy

 v

 c
I
V
• Example Problem 2
– As measured in the laboratory on earth a spectral line of
Hydrogen has a wavelength = 656.285 nm. But in the
spectrum of the star Vega this line has a wavelength of
656.255nm. Calculate the speed at which Vega is moving
relative to Earth.
– Is Vega moving towards or away from Earth?

 v
v  ( )( c )

 c

v  13,713.554m / s
R
O
Y
G
B
I
V
Earth Lab
moving towards earth
Galaxy

Classwork
• Do #7 (speeds only – for now) &
9ab
• page 541 of Tsokos