PURPOSE: to study the many different types and morphological classification of galaxies

MATERIALS: calculator, 7 galaxy images, Hubble Deep Field Image

INSTRUCTIONS: complete your prelab, print out these pages, and complete the activities below

2.1. (4 pts) Sketch the Hubble tuning fork diagram, marking which are elliptical, lenticular, spiral (and barred spiral), and irregular.

2.2. (2 pts) The galaxy M110 has a measured semi-minor axis of b = 11 arcminutes and a measured semi-major axis
of a = 22 arcminutes. What is the elliptical (En) classification for M110?

2.3. (3 pts) What is the eccentricity for M110?

2.4. (1 pt) If the semi-major axis of M110 became smaller, would its En classification get larger or smaller?

2.5.   The galaxy NGC 4302 has a semi-major axis a = 5.5 arcminutes and a semi-minor axis of b = 1 arcminute.

a) (3 pts) What is the galaxy’s inclination?





b) (1 pt) Is this galaxy more "face-on" or more "edge on"?

c) (1 pt) If the semi-major axis of NGC 4302 got larger, would the inclination get larger or smaller?

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**EXPERIMENT**
Practice classifying different galaxies by their morphological type using online activities.

First, obtain the standard Hubble Classification Scheme handout. Examine it and compare it to the descriptions given for
each Hubble type in the lab text, Section 2. Be sure you understand why each galaxy has been given the Hubble type it
has. When you feel comfortable with the Hubble classification scheme, then open this webpage. Read the introduction
and press 'Next' when you are ready to proceed. Follow the directions given on each page.

2.6. (1 pt) The online activity will ask you to classify 10 galaxies by clicking on the 'Your Choice:' button and selecting
a type. Follow the instructions and select a type for all 10 galaxies. When you are finished, a screen stating "So, here's how you did:" will appear. At this point, HAVE YOUR TA COME CHECK OFF THAT YOU HAVE WORKED THROUGH THIS EXERCISE AND GOTTEN TO THIS PAGE. If you did not do well, start the activity over and analyze your selections for each galaxy so you
have a better understanding of Hubble types. Practicing galaxy classification with this applet now will allow you to get a better grade on later exercises in this lab.

View the Princeton University "Galaxy Catalog" to see even more gorgeous galactic images.

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**EXPERIMENT**
The above exercises were for practice. Now let's officially classify some galaxies.

Open this webpage. Be sure to have your standard Hubble Classification Scheme handout on hand for reference.

2.7. (15 pts) For each galaxy on your screen, compare to what you've learned and the standard
Hubble Classification Scheme handout to determine its specific Hubble type. When you are certain, print out this page and record your answers. Be sure to turn in this page with your lab activities.

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**EXPERIMENT**
Use your standard Hubble Classification Scheme handout to classify the 7 galaxy images provided by the TA.

2.8. (7 pts) Match each of the following galaxies with their corresponding Hubble type.

a) M31    __________

2.9. (2 pts) Draw a Hubble tuning fork diagram with the 7 galaxies from #2.8 in it:

3.1. (1 pt) Galaxy X has a radius of R = 15 kpc. Which of the four general galaxy types could this system possibly be
classified as?

3.2. (1 pt) Galaxy X has a mass of M = 1 x 10¹¹ M_SUN. Based on this, which of the four general galaxy types could this
system possibly be classified as?

3.3.   The radius of Galaxy X in units of kilometers is R = 6.172 x 10¹⁷ km. From #3.2, we know that its mass is M = 1 x 10¹¹ M_SUN. Recall the constant G = 6.67 x 10^-20 km³ / kg · s²
and the M_SUN = 1.99 x 10³⁰ kg.

a) (3 pts) Use the virial theorem equation to find the galaxy's rotational velocity (in km/s).








b) (1 pt) Based on your answer to #3.3a, which of the general galaxy types could this system possibly be?

3.4.   Galaxy X has a luminosity of L = 6.4 x 10⁹ L_SUN.

a) (3 pts) Find the galaxy’s mass-to-light ratio.







b) (1 pt) Based only on your result for #3.4a, which of the four general galaxy types could this system be classified as?

3.5. (1 pt) Based on your results from questions #3.1, #3.2, #3.3b, #3.5, which of the four general galaxy types can
this system be?

3.6. (1 pt) If Galaxy X has an HI gas content of about 0.7%, which one of the general galaxy types is Galaxy X?

4.1. (2 pts) Sketch the RS system tuning fork diagram for galaxy clusters, labeling each type.

4.2. (3 pts) Examine these two graphs of Local Group data

a) List the 4 general galaxy types (first graph) in order from the most to least common in the Local Group.



b) Which specific Hubble type (second graph) is the most common in the Local Group?


c) Which specific Hubble type (second graph) is the least common in the Local Group?

4.3.   Look at this image of a cluster of galaxies called the Coma Cluster. Every object in this image is a galaxy
(except for the three white objects with diffraction spikes coming out of them ... these are stars).

a) (1 pt) Estimate the number of each galaxy type in this cluster:

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**EXPERIMENT**
Let's estimate the total number of galaxies in the Universe using the Hubble Deep Field images.
Obtain a Hubble Deep Field image of either Camera A, B, or C. Notice the image is divided into 12 regions.

4.4. (1 pt) One side of the Hubble Deep Field image covers a region about 1.667 x 10^-2 degrees. The area of the
entire picture is measured in square degrees, so we take the size of one side and square it:

4.5. (3 pts) Note that there are 4.13 x 10⁴ square degrees in the entire sky. How many HDF pictures you would
need to cover the whole sky?

4.6. (3 pts) Finally, we stated earlier that it took 10 days to obtain 1 picture. How long (in days) would it take to
cover the whole sky with similar pictures?

Clearly, this is a very long time! Therefore, astronomers can only take pictures of small fractions of the sky at one
time. It is not practical to try and count the total number of galaxies in the Universe in this manner. Instead, we will
count the galaxies on the one Hubble Deep Field image and estimate the total number in the Universe by assuming
that the Hubble photo is representative of the entire sky.

4.7. (4 pts) Take a look at the Hubble Deep Field image. Count up all the galaxies in each grid box and total it to
get an estimate of the total number of galaxies in this field:

4.8. (3 pts) Now estimate the total number of galaxies in the whole sky, using our previous answer for the number
of pictures it would take to cover the whole sky:

This is an amazingly large number! And considering our calculation of the size of the Milky Way, it makes us feel
pretty small. With 200 – 300 billion stars in each galaxy, that makes the number of stars in the entire Universe
impossible to comprehend! It makes you wonder how many other solar systems like ours there may be? But maybe
even more amazing, is that humans have evolved to the point where we can even make this estimation at all! Now
let’s look at our calculation a little more carefully for errors…

4.9. (2 pts) Remember, our calculations rested on the assumption that the Hubble image represents all areas of
the sky. Therefore there may be systematic errors in our estimation. We will estimate the accuracy of our result by
using the image regions (from question #4.7) with the smallest and highest numbers of galaxies:

Region with smallest galaxy count : ____________ Count : ____________

Region with largest galaxy count : ____________ Count : ____________

4.10. (4 pts) Using the same math as question #4.8, estimate the total number of galaxies in the Universe using the
smallest and largest galaxy counts. Note, that this count is only 1/12 of the whole image, so you will need to multiply
each calculation by 12 to get the final answers.

smallest count region:






largest count region:

4.11. (3 pts) That is some error! But that’s expected when you are dealing with objects so far away and making so
many assumptions in order to perform the calculations. List at least 2 assumptions we made. How may they have
affected our results?

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