PURPOSE: to study the size, structure and components of our Galaxy – the Milky Way

MATERIALS: calculator, graph paper, galactic globe pattern, colored markers, scissors, skewer

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

2.1. (1 pt) A galaxy is defined as :

 a)   a diffuse cloud of gas and dust
 b)   a gravitationally bound system of stars, gas, and dust
 c)   a dense spherical cluster of tens - hundreds of thousands of stars
 d)   a large system of planets orbiting a star

2.2. (3 pts) Match each structural component of a spiral galaxy with its appropriate location on this galaxy diagram.

2.3. (3 pts) The Milky Way galaxy is shaped like a disk. It’s radius is approx. r = 1.5 x 10⁴ pc. If we assume it is
circular, the area is given by A= π r². What is the area of the Galaxy’s disk?

2.4. (3 pts) The height H of the Milky Way’s disk is approx. h = 600 pc. The volume is given by V= A x h. What is
the volume (units of pc³) of the Milky Way?

2.5. (2 pts) The average distance between stars is taken to be 1 pc in all directions, or 1 cubic pc (pc³). How many
stars at most then could fit inside the Galaxy?





2.6.   In this calculation we have made some assumptions, making your result a high estimate of the number of stars
in the Milky Way. The actual count is believed to be 200 - 250 billion stars!

a) (2 pts) What were the two main assumptions we made in performing our calculation?







b) (1 pt) How does your result in #2.5 compare to the actual estimated number of stars?

2.7. (3 pts) The Earth, in units of parsecs, has a diameter of 4.13 x 10^-10 pc. The Milky Way galaxy has an average
diameter of 50,000 pc. How many Earths could fit across it?

Wow! That is definitely a lot of Earths to fit inside the Galaxy. Now you can see why astronomy is such a complex
and interesting subject, requiring good telescopes, a lot of time, and a lot of patience. It is gigantic!

3.1. (4 pts) For each galactic sphere component listed below, record the letter associated with its correct location on the
galactic sphere diagram:

celestial equator
galactic equator
North Galactic Pole
South Galactic Pole

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**EXPERIMENT**
Now we will visualize galactic coordinates by making a model.

3.2. (1 pt) Look at the galaxy disk image. A large white arrow points to where our Solar System is located in the
Milky Way. All other objects are viewed from this point. What
are the galactic coordinates of our Sun?

3.3. (3 pts) For each of the following objects determine whether it is closer to the center or edge of the Galaxy and
which structural component (bulge, disk, halo) it is in. [HINT: remember that all objects are viewed from our Solar System (white arrow).

b	l	CENTER/EDGE?	COMPONENT?
-5°	283°
+76°	32°
+11°	4°

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**EXPERIMENT**
Map Milky Way objects to find the direction to the center of the Galaxy.

For this exercise you will need the GC Distribution graph from this website. Print out the specialized graph. Since the
bulge of the Galaxy formed first and is oldest, mapping the oldest galactic objects tells us approximately where the
center of the Galaxy is.

3.4.   Below is a table of data for old star clusters found in the Milky Way :

CLUSTER	LONG (l)	PROJ. DIST (kpc)		CLUSTER	LONG (l)	PROJ. DIST (kpc)
NGC 104	306	3.5		NGC 6341	68	6.5
NGC 288	147	0.3		NGC 6356	7	18.8
NGC 362	302	6.6		NGC 6366	18	16.7
NGC 2808	283	8.9		NGC 6535	27	15.3
NGC 4147	251	4.2		NGC 6638	8	15.1
NGC 4590	299	11.2		NGC 6712	27	5.7
NGC 5272	42	2.2		NGC 6752	337	4.8
NGC 5634	342	17.6		NGC 6760	36	8.4
NGC 5694	331	27.4		NGC 6864	20	31.5
NGC 5904	4	5.5		NGC 6934	52	17.3
NGC 6121	351	4.1		NGC 6981	35	17.7
NGC 6144	352	16.3		NGC 7089	54	9.9
NGC 6229	73	18.9		NGC 7492	53	15.8
NGC 6235	359	18.9		Pal 5	1	24.8
NGC 6266	353	11.6		Pal 10	53	8.3
NGC 6273	357	7		Pal 11	32	27.2
NGC 6287	0	16.6		Pal 12	31	25.4
NGC 6333	5	12.6		O 1276	22	25

a) (6 pts) Using the special graph paper provided, plot the star cluster data from the table above. Note that the
Sun is located at the center of the graph, with each consecutive ring representing the projected distance in kpc
(i.e, 5, 10, etc.). Longitude (l) values are labeled along the perimeter of the circle. Be sure to turn this page in
with the rest of your lab activities.

b) (2 pts) Describe the distribution of these objects in our Galaxy as seen from Earth?








c) (2 pts) How does this plot prove that our Sun is NOT at the center of the Galaxy (or anything else for that matter)?







d) (2 pts) Estimate the center of the distribution of clusters and mark/label it on your graph. Towards which constellation is the direction of the center of the Galaxy?

3.5. (3 pts) Here is true distance (D) data for 20 prominent clusters. If these clusters were randomly distributed around the Galaxy early on in its formation, then the average value of D would be the Earth’s distance (in kpc) from the Galactic center. Calculate this and show your work below :

CLUSTER	D (kpc)		CLUSTER	D (kpc)
Reticulum	0.4		NGC 5053	2.7
AM 4	17.3		NGC 5272	1.5
Pal 5	18.6		NGC 5466	3.0
Pal 12	10.3		NGC 5634	13.2
Pal 13	0.9		NGC 5824	19.4
NGC 288	0.1		NGC 6229	5.8
NGC 1466	3.0		NGC 6864	15.3
NGC 1841	16.1		NGC 6981	11.9
NGC 4147	1.0		NGC 7006	14.4
NGC 5024	2.7		NGC 7492	7.8

4.1. (1 pt) Population I stars are __________ objects; Population II stars are __________ objects :

 a)   young/disk ; old/halo
 b)   young/disk; young/halo
 c)   old/halo; young/disk
 d)   old/halo; young/halo

4.2. (4 pts) Go to this website. Look at image #AAT70 (globular cluster NGC 5094) and #AAT10 (open cluster
NGC 3293). Compare and contrast these two star clusters by describing the shape, size, number of stars, and
distribution for each.

4.3. (3 pts) The open cluster NGC 3293 is believed to be about 10 million years old. It is comprised of at least 50
young stars, the hottest being of spectral type B0 (T=25000 K). Which of the four major gas/dust galactic regions
is this open cluster likely to be found? Give two reasons why.

4.4.   The globular cluster M5 has an apparent magnitude m = 5.6 and a distance of d = 7.52 x 10³ pc.

a) (3 pts) Use the DM equation, M = m + 5 – 5 log d, to find the absolute magnitude M of this globular.








b) (1 pt) The Milky Way galaxy has near 200 globular clusters with an average M = -7.4. Is the globular cluster
M5 brighter or dimmer than the average Milky Way globular cluster?


c) (2 pts) If the average absolute magnitude for the brightest individual stars in the Galaxy is M=0, explain how then that globular clusters are vital to studying Galactic structure.

4.5. (1 pt) List the following objects in order of size from the smallest to the largest: a globular cluster, the Sun, the Universe, the solar system, an open cluster, the Milky Way, the Earth.

4.6. (2 pts) The graph you made in #3.4 and a 1-D graph of D (that you could make from the data in #3.5) both provide only an incomplete visualization of the Galaxy. What information about the distribution of objects can you infer from these graphs? What information is NOT able to be determined from these graphs?

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**EXPERIMENT**
This is a guided exercise for the entire class in three dimensional mapping.

To help develop your understanding of 3-D mapping, you will physically make a map. First, print out this worksheet.
Now, with the guidance of your TA select a location in the classroom and then decide if you will be sitting on the floor,
sitting in a desk chair, sitting in a lab stool, standing, or standing on the desk. Now, your TA will allow several people
to take turns becoming the “vantage point” (i.e., the Earth). Take note of what the vantage point may see in each of the situations your TA arranges. Ask your TA for a turn if you have any trouble with the exercises below.

Look at your worksheet. For simplicity, assume that the top of each map represents the ceiling and the map bottom is
the floor. In all problems, the "vantage point" is sitting in a lab stool. Therefore, people who are sitting on the floor or
in desk chairs are considered below you and people standing up or on desks are considered above you. Assume that
the left-most segment of the map is when you are looking towards the front of the room, and that segments to the right
move in a clockwise fashion around the room (so that the end of the right-most segment is representing the front of the
room also). Remember that we are imagining that you don’t know anything about the distance to any of your classmates,
only what direction they are in.

4.7. (2 pts) For the 1st worksheet map, draw the distribution if you are at the center of a uniform sphere of classmates.

4.8. (2 pts) For the 2nd map, draw the distribution if you are at one edge of a uniform sphere of classmates.

4.9. (2 pts) For the 3rd map, draw the distribution if you are at the center of a thin disk (i.e., everyone sitting
in any type of chair or standing) of classmates.

4.10. (2 pts) For the 4th worksheet map, draw the distribution if you are at one edge of a thin disk of classmates.


*Be sure to turn in this page with the rest of your lab activities when you are finished.

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**EXPERIMENT**
Now will we construct a celestial object map and use it to learn more about the Galaxy’s structure and our place in it.

Obtain a galactic globe pattern worksheet. It is a blank map of the sky with all the constellation boundaries marked on
it. This map is essentially a galactic celestial sphere with the Earth located at the center. Also examine this data table.
Print out this data page. It lists numerous constellations within which many bright objects can be observed with the
naked eye or a small telescope. For the different constellations, the table indicates the quantity of open clusters, globular clusters, nebulae, and galaxies that are found there.

Map the objects on the globe pattern by following these instructions:

Step 1: Assign each of the four celestial objects (open cluster, globular cluster, nebulae, or galaxy) a different color of marker. On your data table, complete the legend indicating which colors correspond to which object types.
Step 2: For each constellation that contains objects in a category, place that many dots in that constellation’s boundaries.
For example, Andromeda has 3 galaxies so you would place three dots (with the color assigned to galaxies) in that box.
Step 3: Now, cut out your map by cutting along the outside boldface lines. Be sure NOT to cut off the tabs at the top
and bottom of each map section. They are essential in constructing your celestial sphere.
Step 4: Obtain a wooden skewer. Note that each map section tab has a circle on it. Carefully stick the skewer through
the circle in the bottom tab of the first map section. Continue this for each map section so that the tabs overlap.
Step 5: Now carefully stick the skewer through the circles of the top tab of the map sections in the same manner. When finished, use some tape to secure all the tab ends together. Then slide the skewer out from the globe.

**NOTE: If you need to save time, you may opt to break up into groups of four, allowing each group member to plot one color (i.e., object) on their globe. When everyone is done, just share and compare all four globes. Be sure to turn them in with the rest of your
lab activities.

4.11. (2 pts) Describe the distribution of open clusters and nebulae on your globe.

4.12. (1 pt) Describe the distribution of globular clusters on your sphere.

4.13. (1 pt) In which constellation(s) is the largest number of globular clusters?

4.14. (2 pts) Based on their defining properties, are the open and globular clusters distributed as you might expect?

4.15. (4 pts) Do any of your descriptions of the firefly distributions in questions #4.7 - 4.10 match any of the object distributions on your globe? If so, which ones and explain how.

4.16. (3 pts) Based on all you learned in this lab, where do you think we (Earth) are located within the Milky Way
Galaxy (i.e., center, edge, top, bottom, etc.)? Give at least two reasons to support your answer.

4.17. (6 pts) Draw a sketch of the Milky Way (edge-on view). Label the three galaxy components and mark the
location of our Sun. Also be sure to mark and label the four types of objects, showing where they are primarily
found with respect to our Galaxy.

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This web page created and maintained by Andrea Folcik