Hubba Hubba Hubble
Integrated Science
Atomic spectra can tell us many things about celestial objects, including composition, distance,
and temperature. In 1929, Edwin Hubble proposed that the spectra also tell us about the motion
of the universe. He explained that the differences in wavelength of spectra from distant sky
objects, called the red-shift, is due to the Doppler effect. In the Doppler effect, wave properties
of length and frequency appear different because the motion of the object compresses or
increases the distance between successive waves. By determining the direction and degree of
shift between expected and apparent spectral wavelengths, the distance and the velocity of a farsky object can be determined. Hubble concluded that the difference in the spectral lines of
galaxies occurs because the universe is expanding.
1.
Highlight in color the lines of the EXPECTED hydrogen spectra, based upon the
wavelength position of each. (Use your atomic spectra lab as reference.)
2.
Determine the wavelength of the longest wavelength spectral line (a) of Hydrogen
from the scale. Record. Repeat for lines ‘b’ and ‘c’. Record in a data table.
3.
Uisng a scale of 2mm = 1nm, determine the wavelengths for a, b, and c of
Andromeda galaxy. Record. Use these values to determine the frequency shift for
each line as follows:
Frequency shift = observed position – expected position
Record.
4.
Find the redshift for each line by dividing the frequency shift by the expected
position of the line. Record.
Redshift =
5.
Find the average red-shift for Andromeda. Record.
Average redshift =
6.
Frequency shift
----------------------Expected position
(rs ‘a’) + (rs ‘b’) + ( rs ‘c’)
-------------------------3
Repeat 3-5 for Triangulum galaxy.
7.
Determine the velocity (in km/sec) of each galaxy based on their average red-shift
multiplied by the speed of light, c, a value of 300,000km/sec.
V = c (average redshift)
Record.
8.
Since a galaxy’s distance is proportionate to its velocity, the distance of the
Andromeda galaxy can be predicted by dividing it’s velocity by the Hubble constant
(H0). Current evidence points to a Hubble constant close to 74 km/sec/megaparsec
(Mpc). Use this H0 value to find the distance to the galaxy in Mpc.
d
=
V
---------------H0
9.
Repeat steps 7 - 8 for the spectra from the Triangulum galaxy.
10.
Create a graph comparing both velocity and distance against red shift for the two
galaxies.
-------------------------------------------------------------------------------------------------------------Follow-Up:
1. What can you infer about the frequencies of the light coming from these distant galaxies as
they recede from us, given the changes in wavelength?
2. Consider the colors of the shifted spectral lines for both galaxies. Is the color of each spectral
line the same as expected? Why or why not?
3. How does the Doppler effect explain this apparent change in wavelength and frequency?
4. The light from these galaxies is not visibly more red - why then the name ‘red-shift’?
What form of EM would the most distant (and therefore most red-shifted) light be in?
5. How old is the light from Andromeda and Triangulum? Convert from Mpc to Light Years
(1Mpc = 3.26 x 106 ly).
Calculator at: http://astronomy.swin.edu.au/~elenc/Calculators/redshift.php
6. The Hubble constant uses the unit of km/sec/megaparsec. As you saw in question 5, Mpc can
be converted to Light years, a time unit. What does this unit – with two time elements – tell you
about the rate at which the universe is expanding? (Hint: consider the unit for the rate bodies fall
to earth from last unit).