Lesson 3: Examining Spectra of Three Galaxies
Overview:
In this activity, students look at different forms of spectra (continuous, emission
and absorption) and differentiate between them / their causes. Following this, the students
examine sample spectra from the three featured galaxies (NGC 3187, UGC 11816 and the
Toverball Galaxy), and note that common emission peaks have moved.
Learning Cycle Stage:
Time:
Engage, Explore, Explain
45+ minutes
Learning Objectives:
The teacher will be able to assess student’s prior knowledge about redshift and spectra. (This
activity assumes some previous experience with emission spectra. If students are unfamiliar with
spectra, we recommend a supplementary activity to be completed before Lesson 3).
Students will be able to compare spectra of galaxies and note differences/similarities. Students
will be able to explain redshift, determine relative redshift, and understand the relationship
between redshift and velocity.
Students will be able to sequence galaxies based on velocity and direction.
What You Need:
For the teacher:
□ Large printed or LCD projected image of Figure 3.1-3.6, Image 1.1, 1.4, 1.5, 1.7 and 3.13.3
For each team of 2-4 students:
□ 3 galaxy images, set to scale- so true apparent size can be seen (Image 3.1-3.3)
□ Copies of the spectra of each of the three selected galaxies (Figure 3.5-3.7)
Getting Ready:
□ Prepare LCD projector to project Figure 3.1-3.6, Image 1.1, 1.4, 1.5, 1.7 and 3.1-3.3
□ Print (in color) one set of three color images (Image 3.1-3.3) for every 2-4 students.
Laminate if desired. **Keep these after the lesson, as they will be re-used in Lesson 5.
□ Print (in black/white) one set of three galaxy’s spectra (Figure 3.4-3.6) for every 2-4
students. **Keep these after the lesson, as they will be re-used in Lesson 5.
□ Download Powerpoint from MyHubbleDiagram.org/visuals [optional]
*If students are unfamiliar with spectra, we recommend that teachers complete an activity
with their students that involves observing various emission tubes through diffraction
gratings. This should be done before proceeding with Lesson 3. A good example of this
activity can be found at
http://www.pbs.org/newshour/extra/teachers/lessonplans/science/hubble.html
Comparing Spectra
1. Hold up (or use and LCD projector to show) the same image of M51 (Image 1.1) shown
in Lesson 1 and/or Images 1.4, 1.5 and 1.7. Ask: How might astronomers want to
study these galaxies? What do they, as students, want to know about the galaxies
(for example: composition, distance, size, brightness)?
2. Point out that astronomers cannot travel to the galaxies, and therefore they can only study
them by looking at the light. Ask your students to consider something they may have
done in a Chemistry class: how have they used light to determine composition? (If
students have prior experience with emission tube, ask them to recount the experience).
3. Hold up (or LCD project) Figure 3.1 (3 types of spectra). If students are already familiar
with spectra, ask the following questions:
a. What is a continuous spectrum? How is it produced?
b. What does it mean to have a dark line or a bright line?
*If students are unfamiliar with spectra, we recommend that teachers complete an activity
with their students that involves observing various emission tubes through diffraction
gratings. A good example of this activity can be found at
http://www.pbs.org/newshour/extra/teachers/lessonplans/science/hubble.html
4. Distribute one of the spectral graphs (any of Figure 3.5- 3.7). Ask: Which of the three
pictures of spectra is being represented by the graph? (If students recognize
immediately that it is emission, go to the next step. If students struggle to relate a
graphical representation of absorption spectra to picture of absorption spectra, try
projecting / showing them Figure 3.2. It may help.)
Figure 3.1: 3 types of spectra
Credit: http://www.astro.princeton.edu/~clark/EveryDopAct.html
Figure 3.2: Graphs of Spectra
Credit: http://www.astro.princeton.edu/~clark/EveryDopAct.html
5. Remind students that in Lesson 2, they saw that wavelengths became shorter as an object
moved toward us, and appeared longer as an object moved away from the observer. AskIf we were to look at spectrum of an object that is moving away from us, how would
the spectrum be different? (A: Waves are shorter/blue shifted when the object is
moving toward the observer, and longer when the object is moving away from the
observer).
6. Show Figure 3.3. Note what has happened to the absorption lines in this distant
galaxy. Do you think it is redshifted or blueshifted?
Sun
Distant
Galaxy
Note new wavelength for same absorption line
Figure 3.3: Redshifted Galaxy
Credit: http://www.astro.princeton.edu/~clark/EveryDopAct.html
Figure 3.4: Compression of waves due to movement
Credit:
http://www.iop.org/activity/education/Teaching_Resources/Teaching%20Advanced%20Physics/Astronomy/Astrophysi
cs/page_5423.html
7. Ask- If we were to look at spectrum of two objects that are moving away from us at
two different speeds, how would the spectrum of a faster moving object be different? If
we were to look at spectrum of two objects that are moving toward us at two different
speeds, how would the spectrum of a faster moving object be different?
8. Distribute sets of 3 spectra (Figure 3.5-3.7). Note that the same major emission peaks
have been identified on each. Give the rest wavelength for H-alpha (6560Å) and H-beta
(4860 Å). Remind students that if the peak is shifted to longer wavelengths from its rest
wavelength, it is a sign that the galaxy is red-shifted and moving away from us.
9. Ask: Based on what you see in the spectrum, what can you infer about the motions
of each galaxy? (They should be able to describe the galaxies’ direction of motion, and
be able to rank them in order of redshift/velocity). [All of the galaxies are redshifted. The
fastest moving galaxy / most redshifted is Toverball, followed by UGC 11816, and the
slowest moving is NGC 3187. The approximate wavelengths for the H-alpha and H-beta
lines in each graph are listed below.].
Least shift
NGC3187 – H-alpha at ~6600A, H-beta at ~4890A
UGC11816 – H-alpha at ~6670A, H-beta at ~4950A
Most shift
Toverball – H-alpha at ~7220A, H-beta at ~5340A
10. Tell: The more that the H-Alpha line moves toward longer, redder wavelengths, the
greater the redshift, and the faster it is moving away from us. The more that it moves
toward the shorter, bluer wavelengths, the greater the blueshift, and the faster that the
galaxy is moving toward us. Remind students of the applet they saw yesterday (or show
the students again).
11. Question to Ponder for Tomorrow: Which of the three galaxies is moving fastest?
12. Collect the Images and Spectra for re-use in Lesson 5.
Teacher Hints:
-Suggest that students measure and record the difference in wavelength of the H-alpha and
H-beta peaks in one graph. They should see that the difference between the two wavelengths
– regardless of graph -is approximately 1700Å.
-Ask the students to consider if this difference changes for galaxies with different redshifts.
Students should be able to determine that the galaxies with the higher redshift are moving
fastest, but they won’t yet realize that this is also related to distance. Don’t tell them this
yet!! Shhhhhh.
Image 1.1:
M51
Image 1.4
Image 1.5
Image 1.7
Image 3.1: SDSS image of UGC 11816
Apparent Magnitude 14.9
Image 3.2: SDSS image of NGC 3187
Apparent Magnitude: 13.96
Image 3.3: SDSS Toverball Galaxy
Apparent Magnitude 18.37
Figure 3.5: Spectra of NGC 3187
Rest Wavelengths:
H-alpha = 6560 A
H-beta = 4860 A
Spectra of NGC 3187
100
90
Hα
80
70
Flux
60
50
40
Hβ
30
20
10
0
4500
5000
5500
6000
Wavelength (Angstroms)
6500
7000
7500
Figure 3.6: Spectra of Toverball Galaxy
Rest Wavelengths:
H-alpha = 6560 A
H-beta = 4860 A
Spectra of Toverball Galaxy
50
Hα
45
40
35
Flux
30
25
20
Hβ
15
10
5
0
4500
5000
5500
6000
Wavelength (Angstroms)
6500
7000
7500
Figure 3.7: Spectra of UGC 11816
Rest Wavelengths:
H-alpha = 6560 A
H-beta = 4860 A
Spectra of UGC 11816
120
Hα
100
Flux
80
60
Hβ
40
20
0
4500
5000
5500
6000
Wavelength (Angstroms)
6500
7000
7500
Figure 3.8: Spectra of all three Galaxies on the same graph
Comparison of H-alpha and H-beta Peaks for all Three Galaxies
120
UGC 11816
NGC 3187
Toverball
100
Flux
80
60
40
20
0
4500
5000
5500
6000
Wavelength (Angstroms)
6500
7000
7500