Lesson 5: Examining Images and Spectra of Three
Galaxies
Overview:
In this activity, students examine the images and spectra of the three featured
galaxies. They will investigate their relative distances and sizes based on basic assumptions
about galaxies.
Learning Cycle Stage:
Time:
Engage, Explore, Explain
2 x 40 minutes (allow more time for students with lower math skills)
Learning Objectives:
Students will be able to compare spectra of galaxies and relate the spectra to distance / redshift.
Students will be able to relate relative/apparent size and brightness of galaxies to their
approximate distance.
Students will begin to calculate redshift and velocity.
What You Need:
For each team of 2-4 students:
□ 3 galaxy images, set to scale- (Same images from Lesson 3: Image 3.1-3.3)
□ Spectra of each of the three selected galaxies (Same Figures from Lesson 3: Figure 3.53.7)
□ One worksheet
□ One Scientific Calculator
Getting Ready:
□ Prepare LCD projector to project Figure 3.5-3.7 and Images 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. Note that these were already printed in Lesson 3- no need to reprint
if you still have those copies.
□ Print (in black/white) one set of three galaxy’s spectra (Figure 3.5-3.7) for every 2-4
students. Note that these were already printed in Lesson 3- no need to reprint if you still
have those copies.
□ Print one worksheet per student.
□ Download Powerpoint from MyHubbleDiagram.org/visuals [optional]
Images and Spectra of 3 different Galaxies
1. Pass out the sheet (Page 34) that shows all three galaxies at true relative apparent size
(Images 3.1-3.3) and their spectra (Figures 3.5-3.7). Handout student worksheet and
allow students to work independently on the sheet.
2. Discuss answers.
3. Extension discussion: Look at the speed of each of the galaxies and compare these
speeds to the speed of light. Those galaxies are moving VERY fast!
Student Worksheet: Examining Images and Spectra of
Three different Galaxies
Examine the spectra and images of NGC 3187, UGC 11816 and The Toverbal Galaxy. They are
labeled Image 3.1, 3.2 and 3.3 and Figure 3.5, 3.6 and 3.7. Answer the following questions:
1. Look at the spectra (Figure 3.5, 3.6 and 3.7). Find the labeled H-alpha line in each graph.
For a galaxy that is not moving relative to us, this line is observed at 6560Å.
Astronomers call this the rest wavelength. Notice that for these three galaxies, the Halpha line has shifted to longer wavelengths (shifted toward the red). In which galaxy is
the line shifted most toward the red?
Least shift
Most shift
______________________
______________________
______________________
2. If the amount that the H alpha line has been shifted to the red is proportional to how fast
it is moving away from us, rank the three galaxies from slowest to fastest.
Slowest
Fastest
______________________
______________________
______________________
3. Look at the images of the three galaxies (Image 3.1, 3.2 and 3.3). What is the
relationship between the apparent magnitude of the galaxy and the amount that the Halpha line has shifted to the red (from Question #2)?
4. What is the relationship between the apparent size of the galaxy and the amount that the
H-Alpha line has shifted to the red?
5. Based on what you see in the image, which galaxy would you predict is most distant?
What assumption(s) did you make?
Closest
Farthest
______________________
______________________
______________________
6. If these galaxies were all at the same distance, what could you conclude about their size?
Smallest
Largest
______________________
______________________
______________________
7. If these galaxies were all the same size, what could you conclude about their distance?
Closest
Farthest
______________________
______________________
______________________
Actually, we know that galaxies are generally at very
different distances. Scientists have studied the
different possible sizes of galaxies, and they have
found that some are larger, and some are smaller, but
they are generally all similar in size. In other words,
there is not much difference in size between the
largest galaxies and the smallest galaxies. It is not
really a bad assumption to suppose that the galaxies
you observe are all approximately the same size.
8. If galaxies are all approximately the same size, they should be equally bright. How
would the apparent brightness of distant galaxies compare to those closer to us?
9. So when we assume that every galaxy is the same size and the same brightness, what can
we conclude about a galaxy if it appears dimmer or smaller than another galaxy in an
image?
http://www.iop.org/activity/education/Teaching_Resources/Teaching%20Advanced%20Physics/Astronomy/Astro
physics/page_5423.html
Note that in the image above, each absorption line has been shifted. We can use this shift
to calculate the velocity of the galaxy. The symbol for change in wavelength is 
10. Using Figures 3.5, 3.6 and 3.7, measure the observed H-alpha wavelength for each
galaxy. (You may have already done this in a previous activity).
The rest wavelength for H-alpha is 6560Å. Use the formula below to calculate
velocity for each galaxy.
(Use the speed of light : c = 3 x 108 m/s)
velocity
=
the
observed wavelength – rest wavelength) x c (speed of light)
 (rest wavelength)
Calculation Space:
Velocity of NGC 3187: _________________
Velocity of UGC 11816: _________________
Velocity of Toverball Galaxy: _______________
11. Astronomers use the term Redshift to express the velocity of an object as a fraction of
the speed of light. For example, an object with a redshift of 0.01 is traveling at 1/100th of the
speed of light. Use the following formula* to calculate redshift:
redshift
=
velocity of object
c (speed of light)
Using the data that you calculated in Question 10, calculated the redshift of each galaxy.
(Use c = 3 x 108 m/s)
*The formula can only be used for objects that are traveling at less than a ~1/10th of the speed
of light. If an object is traveling faster, a different formula must be used to account for
relativity. We are not using this formula!
Redshift of NGC 3187: _________________
Redshift of UGC 11816: _________________
Redshift of Toverball Galaxy: _______________
For fun. Compare your calculated values to the ones on the Cosmic Confrontation Cards!
How close are you to the ‘accepted’ value?
12. Finish the following sentences. As the H-alpha line is shifted toward longer and longer
wavelengths, :
-the Velocity of the galaxy will ….
-the Redshift of the galaxy is….
-the Apparent Size of the galaxy is….
-the Apparent Magnitude of the galaxy is….
-the Distance of the galaxy is….
13.
Create a rough graph of the ‘Magnitude vs Redshift’ of the three galaxies.
Redshift
High
Low
Bright
Faint
Magnitude
Answers to Student Worksheet: Examining Images and
Spectra of Three different Galaxies
1. In which galaxy is the line shifted most toward the red?
Least shift
Most shift
NGC3187 – Halpha at ~6600A
UGC11816 – Halpha at ~6670A
Toverball – Halpha at ~7220A
2. If the amount that the H alpha line has been shifted to the red is proportional to how fast
it is moving away from us, rank the three galaxies from slowest to fastest.
Slowest
Fastest
NGC3187 – Halpha at ~6600A
UGC11816 – Halpha at ~6670A
Toverball – Halpha at ~7220A
3. What is the relationship between the apparent magnitude of the galaxy and the amount
that the H-alpha line has shifted to the red (from Question #2)?
The galaxies with the most H-alpha line shift appear dimmest. The galaxies with the least
shift appear brightest.
4. What is the relationship between the apparent size of the galaxy and the amount that the
H-Alpha line has shifted to the red?
The galaxies with the most H-alpha line shift appear smallest. The galaxies with the least
shift appear largest.
5. Based on what you see in the image, which galaxy would you predict is most distant?
Closest
NGC3187
UGC11816
Farthest
Toverball
I made the assumption that all galaxies are equally bright and the same size. So if you
put the same object at a greater distance, it would appear smaller and dimmer.
6. If these galaxies were all at the same distance, what could you conclude about their size?
Smallest
Largest
Toverball
UGC11816
NGC3187
7. If these galaxies were all the same size, what could you conclude about their distance?
Closest
NGC3187
UGC11816
Toverball
Farthest
8. How would the apparent brightness of distant galaxies compare to those closer to us?
If all galaxies are all approximately the same size and brightness, then closer galaxies
should appear much brighter and more distant galaxies would appear dimmer.
9. So when we assume that every galaxy is the same size and the same brightness, what can
we conclude about a galaxy if it appears dimmer or smaller than another galaxy in an
image?
We could then assume that the galaxy must be farther away than others that appear
larger and/or brighter.
10. Using Figures 3.5, 3.6 and 3.7, measure the observed H-alpha wavelength for each
galaxy. (You may have already done this in a previous activity).
The rest wavelength for H-alpha is 6560Å. The speed of light : c = 3 x 108 m/s)
velocity
=
the amount the line is shifted)
 (rest wavelength)
Calculation Space:
NGC3187 – Halpha at ~6600A
=
6560)
 (6560)
x
c (3 x 108)
x
c (speed of light)
=
1,829,268 m/s or 1.8 x 106 m/s
UGC11816 – Halpha at ~6670A
=
6560)
 (6560)
=
5,030,487 m/s or 5.0 x 106 m/s
x
c (3 x 108)
Toverball – Halpha at ~7230A
=
6560)
 (6560)
=
30,182,927 m/s or 3.0 x 107 m/s
x
c (3 x 108)
Velocity of NGC 3187: 1.8 x 106 m/s
Velocity of UGC 11816: 5.0 x 106 m/s
Velocity of Toverball Galaxy: 3.0 x 107 m/s
** Note these numbers are just based on a very rough estimate of the Halpha line position in
the graph, so they are not an exact match to the current accepted values of the object’s
velocity. They are a reasonable estimate though.
11. Astronomers use the term Redshift to express the velocity of an object as a fraction of
the speed of light. For example, an object with a redshift of 0.01 is traveling at 1/100th of the
speed of light. The use the following formula* to calculate redshift:
redshift
=
velocity of object
c (speed of light)
Using the data that you calculated in Question 10, calculated the redshift of each galaxy.
(Use c = 3 x 108 m/s)
Redshift of NGC 3187: 6 x 10-3
Redshift of UGC 11816: 1.7 x 10-2
Redshift of Toverball Galaxy: 1 x 10-1
**Look back at the Cosmic Confrontation cards! These redshift numbers are not that far off
from the accepted values for NGC3187 and UGC11816!
12. Describe how the following galactic characteristics are related to the shift of the H-alpha
line toward longer wavelengths:
-Velocity
-more shift of the H-alpha line = higher velocity
-Redshift
-more shift of the H-alpha line = higher redshift values
-Apparent Size -more shift of the H-alpha line = smaller apparent size
-Apparent Mag. -more shift of the H-alpha line = dimmer apparent Mag.
-Distance
13.
-more shift of the -Halpha line = farther object
Create a rough graph of the magnitude versus the redshift of the three galaxies.
Students should indicate a line that starts in the lower left (low and bright) and moves to
the upper right (high and faint).