Name _____________________________________ Date ____________ Section _____________
Classification of Galaxies1
Hubble’s Tuning Fork was once thought to represent the evolutionary sequence, with galaxies
starting out as ellipticals and evolving to having more structure as they aged into spirals (top of right
fork) or barred spirals (bottom of right fork). In this activity we investigate the defining
characteristics of the different types of galaxies (ellipticals versus spirals) and work to understand
the types better by studying the ages of stars, the amount of dust present, and gas and star formation.
Step 1
Classifying galaxies
Table 1. The galaxies shown in order of the World Wide Telescope tour in the planetarium.
Galaxy ID
Constellation
M87 (NGC 4486)
Virgo
M61 (NGC 4303)
Virgo
NGC 4643
Virgo
M60 (NGC 4649)
Virgo
Defining Characteristics
M88 (NGC 4501) Coma Berenices
M91 (NGC 4548) Coma Berenices
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NGC 4881
Coma Berenices
NGC 4874
Coma Berenices
Inspired by Lecture Tutorials for Introductory Astronomy, 2nd ed., Pearson Education Inc. (2008)
8/4/14
l08_classifying_galaxies.docx
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Step 2
Applying Hubble’s classification scheme based on a galaxy’s morphology
1. Using the images displayed in the planetarium tour, sort the 8 galaxies into either the elliptical or
spiral category. Fill in the table that follows. Be sure to note a few defining characteristics that you
used (shape, color, size, or any other distinctive features).
Hubble’s
category
Galaxy ID
numbers
Elliptical
M87 (NGC 4486)
NGC 4881
NGC 4874
M60 (NGC 4649)
Spiral
M61 (NGC 4303)
NGC 4643
M88 (NGC 4501)
M91 (NGC 4548)
Step 3
Defining characteristics
Understanding the underlying physical characteristics of these types of galaxies
What can the morphology of a galaxy tell us about its physical characteristics? These
characteristics include a. the ages of the stars in the galaxy; b. the presence of dust; and c. the
presence or absence of gas and star formation. All of these properties are linked together in a
physical way. Here we want to find out how these characteristics are related to a galaxy’s
classification.
We start with a review of stars and the H-R
Diagram to answer this question about the
morphology of galaxies. If you were to view
a cluster of stars that contained thousands and
thousands of stars, could you decide what
kinds of stars were dominating the light
coming from each cluster?
Shown at the right is a typical HR diagram
showing all of the possible regions where
stars are found. This diagram could not be
for a single cluster of stars as there would not
be both massive stars AND white dwarfs and red giants in the same cluster. But, we can use it to
infer the kinds of stars dominating the light from a cluster.
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2. You observe a cluster of stars that has many stars radiating blue light. What kind of stars must be
dominating this cluster?
a. white dwarfs
b. massive main sequence stars and blue giants
c. red supergiants and red giants
d. low mass main sequence stars
3. A new cluster of stars has been discovered where all of the stars seem to be reddish-orange (no
star ever appears to be pure red). You conclude that this cluster’s light is dominated by
a. white dwarfs.
b. massive main sequence stars and blue giants.
c. red giants.
d. low mass main sequence stars.
Step 4
The ages of stars in the galaxies
Now, refer to the galaxies numbered 1 - 8 in the images being shown either through slides,
handouts, or a colored version of this document. Assume that the colors shown represent the true
colors of the galaxies. Answer the questions that follow.
4. List the number and classification for each galaxy that appears to be mostly reddish-orange or
orangish-yellow. Using the same star groupings given on the HR diagram above, what is
dominating the light coming from these galaxies?
5. List the number and classification for each galaxy that appears to be mostly blue. Why do you
think these galaxies appear blue?
6. Which type of galaxies (spiral or elliptical) appears to have many young stars as indicated by
their overall color: elliptical, spiral, or both?
7. Do the type of galaxies you identified in question 6 also appear to contain old stars? How do you
know?
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8. Would you state that the galaxies identified in question 4 above contained mostly old or mostly
young stars? Explain your reasoning.
Step 5
Dust in galaxies
Besides stars, galaxies sometimes contain dust. This dust produces dark bands across or patches in
the image of the galaxy.
9. Which of the galaxies show evidence of dust? Note each galaxy’s number and classification.
Step 6
Gas and star formation in galaxies
10. A galaxy that is experiencing active star formation must contain abundant gas. Which type of
galaxy appears to have abundant gas available? What criteria did you use when examining the
images to answer this question?
11. Which type of galaxy (elliptical or spiral or both or neither) is likely to contain all spectral
types of stars, from O to M? Explain your choice.
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