Participation - From Additional Course Pack Materials
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.
Messier 87
Virgo
Messier 61
Virgo
NGC 4643
Virgo
NGC 4643
Virgo
Messier 88
Virgo
NGC 4649
Coma Berenices
Messier 91
Coma Berenices
NGC 4881
Coma Berenices
NGC 4874
Coma Berenices
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 oher distinctive features). Generalize!
Shown at the left 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.
Start with star clusters
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.
Extend our knowledge of stars to galaxies
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?
M87 (NGC 4486)
NGC 4881
NGC 4874
M60 (NGC 4649)
Extend our knowledge of stars to galaxies
5. List the number and classification for each galaxy that appears to be
mostly blue. Why do you think these galaxies appear blue?
M61 (NGC 4303)
NGC 4643
M88 (NGC 4501)
M91 (NGC 4548)
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?
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.
Today’s goals:
Describe the Hubble “tuning fork” and summarize its
use today.
State the basic differences between elliptical and
spiral galaxies in how they look at visible
wavelengths
Be able to classify galaxies based upon their
morphology
Outline the general differences in kinds of stars
dominating each type, the presence of gas and dust,
and the presence or absence of star formation
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Hubble tuning fork
Describe the Hubble “tuning fork” and summarize its use today.
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Elliptical galaxies
Spiral galaxies
M89 E0
State the basic differences between elliptical and spiral galaxies in how they look.
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Spiral galaxies: List at least 3 characteristics.
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State the basic differences between elliptical and spiral galaxies in how they look.
Ellipticals: List at least 3 characteristics.
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State the basic differences between elliptical and spiral galaxies in how they look.
Irregular galaxies are - irregular.
Options:
Spiral
Barred spiral
Elliptical
Irregular
Merging
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Characteristic
Ellipticals
Spirals
Irregulars
These have numbered types ranging from E0 to E7.
This type have disks, arms, central bulges.
Some in this class have bar-like structures across their bulges.
Entire galaxy has stars moving in all possible directions.
Most stars follow nearly circular orbits, traveling in the same direction.
Contain large amounts of dust and cold, dense gas concentrated in disk.
Contain little or no dust and cold gas, just extremely hot X-ray gas.
Active star formation is an on-going event.
Little or no star formation occurring in these galaxies.
These are without symmetry in shape or structure.
These contain primarily old stars (red giants) and thus look orangish.
These have regions of red (hot hydrogen) and blue (massive stars).
These galaxies have been distorted by close encounters or mergers.
Outline the general differences in kinds of stars dominating each type, the presence
of gas and dust, and the presence or absence of star formation
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