Galaxy Classification and Evolution ______/ 40 Lab Grade

Galaxy Classification and Evolution ______/ 40 Lab Grade

Name______Date______Period______

Objective: Classify galaxies based on their appearances and find patterns in galaxy clusters.

Edwin Hubble’s Classification Scheme

Procedure:

• Part 1 Galaxy Classification

Classify ALL of the galaxies on Table 1(S0, Sc, SBa, E0, E4, etc) using the “Galaxies to be classified” packet. Write a short comment of how you arrived at each classification.

Spiralshave a bulge and a disk with a spiral structure. A big bulge with tightly wound armscorresponds to a Sa; a small bulge with loosely wound arms is an Sc – pay attention to the lengthand tightness of the spiral arms and to the relative size of the bulge. Sometimes you might evenbe able to determine intermediate cases; for example anSbc is intermediate between an Sb andSc. If there is a bar, you might have anSBb, or an SBc. Do not confuse spiral arms with tidalarms (a gravitational influence of a close neighbor)

Ellipticalgalaxies tend to be smooth and regular. Ellipticals do not have a disk or a bulge, only aspheroidal component. Their shape somewhat resemble that of a “fuzzy football”. The flattestEllipticals are classified as E7, round ones as E0. In some cases you might see a rather broaddust patch orbiting the galaxy (as in the case of Cygnus A). These dust patches look verydifferent from signatures of dust seen in the arms of spiral galaxies.

Lenticulargalaxies (denoted S0) are often confused with elliptical galaxies, however they arerather different. They look as smooth and regular as Ellipticals, however they have a disk and abulge (Ellipticals do not have a disk). They are basically spiral galaxies with NO arms.

Irregulargalaxies do not have a well-defined shape and are not as symmetrical or regular likeEllipticals or Spirals. Some galaxies may have a rather weird structure – these can be classifiedas “peculiar” or Irr II.

• Part 2 The Colors of Galaxies

Look at the pictures on the COLORED PRINT OUTS to answer the questions on your worksheet in part 2

• Part 3 Galaxy Environments

Galaxies, like stars, exist in clusters (our cluster is known as the Local Group). The morphologies of the galaxies turns out to depend on the environment of galaxies. A nearby cluster, the Virgo cluster of galaxies, is prominent in the sky because of its relative nearness. It lies in the Virgo and Coma Berenices constellations, at a distance of about 65 million light-years, and contains many bright objects. Use the colored images of the Coma Cluster and Hercules Cluster to complete the questions in Part 3.

• Part 4 Galaxy Evolution Look at the Hubble Deep Field. These are galaxies at the edge of the universe. Since light from themtakes a long time to reach us, most of those galaxies are intrinsically young. Take a part of that pictureand count the number of Elliptical, Spiral, and Irregular galaxies. Determine the PERCENTAGES ofElliptical, Spiral, and Irregular galaxies and complete the table and answer the questions in Part 4.

Table 1: Galaxies to be Classified

Name / Type / Comments
M101
M102
M104
M110
M32
M33
M49
M51
M59
M61
M64
M74
M77
M104
LMC
M81
M84
M85
M87
M95
M88
M89
M94
NGC2147
NGC2644
NGC2685
NGC3077
NGC3718
NGC4565
NGC4651
NGC470
NGC5364
NGC5383
NGC55
NGC6946

Part 2 The Colors of Galaxies

Look at the pictures in the packet with the galaxies printed in color.

1. Do the colors of galaxies seem relate to their shapes? Explain.______

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2. Suggest an explanation for the colors of Spiral, Elliptical, and Irregular galaxies. In particular, comment on the amount of gas and dust and the stellar populations of those galaxies. ______

3. Compare the bulge of Spiral galaxies to elliptical galaxies. In what way are they similar? ______

4. Look at M101, NGC2997, and M100. Why are the colors of the bulge and the colors of the spiral arms different? Suggest an explanation. ______

Part 3 Galaxy Environments

Look at the Hercules and Coma Clusters and classify the galaxies roughly according to Elliptical, Spiral, and Irregular.

1. What are the dominant types of galaxies in the Coma Cluster? ______

2. What are the dominant types of galaxies in the Hercules Cluster? ______

Classify 30 galaxies in each cluster and count the number of galaxies in each category. Fill out the table below

Elliptical / Spiral / Irregular / Total Number
Coma / 30
Hercules / 30

3. Determine the percentage of Elliptical, Spiral, and Irregular.

Density / Elliptical / Spiral / Irregular
Coma / very dense
Hercules / not so dense
Poor Groups / few galaxies / 25% / 70% / 5%

4. How does the proportion of Elliptical and Spiral Galaxies depend on the density of the Environment? ______

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5. As the universe ages, galaxies gravitate toward each other, in other words, clusters tend to get denser. What impact could such an evolution have on the morphologies of the galaxies? ______

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Part 4 – Galaxy Evolution

Look at the Hubble Deep Field Image – there are galaxies at the edge of the universe. Take a section of that picture and count the number of Elliptical, Spiral, and Irregular galaxies. Determine the percentages of each in the table below.

% Elliptical / % Spiral / % Irregular
Nearby Galaxies; Observed Today / Coma
Hercules
Poor Groups / 25% / 70% / 5%
Distant Galaxies / Field Galaxies in the HDF

1. How are today’s “field galaxies and galaxies in poor groups” different from distant ones?

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2. Look at the elliptical galaxies in the HDF. Do those Ellipticals look any different from the Elliptical galaxies seen today? What can you infer about the ages of those galaxies? ______

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3. Which types of galaxies have evolved most? Justify your answer. ______

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Conclusion

1. Summarize what you learned about galaxies in this lab (5 complete sentences, or more).

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