WK08_Spiral_Arms
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School
Indiana University, Bloomington *
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Course
A105
Subject
Astronomy
Date
Dec 6, 2023
Type
docx
Pages
5
Uploaded by DrIron12403
Week 8 Exploration
Name:
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A105 – Stars & Galaxies
Spiral Arms
Learning Goals – Students should be able to:
describe how astronomers map out the spiral arms of the Milky Way.
plot and interpret a longitude-distance graph to derive astronomical information.
map star-forming regions in the Milky Way to locate the spiral arms on our side of the
Galaxy.
What is the point here?
Without faster-than-light travel we can't travel up out of the Galaxy to get a top-down view to see
what our galaxy looks like.
Before 1950s, astronomers did not have a direct proof that our galaxy
had spiral structure. We mostly
expected
this to be the case based on seeing that
other
galaxies
that had a disk, dust, and young stars tended to have spiral arms. But even in other galaxies the
spiral structure comes in many different forms. Compare for example, these three spiral galaxies.
Their arms are quite different. How does the Milky Way compare to them?
To provide answers to these question and prove that spiral arms are indeed there, astronomer
invented a new technique in the 1950s that uses radio waves. Like infrared light, radio waves have
wavelength longer than the size of dust particles, so are not impeded by it. With this technique,
astronomers were able to find clouds of gas in which new stars form. The same stars that give rise
to the spiral arms. Importantly, they were able to figure out the distances of these clouds of gas.
So the question is whether these clouds of gas outline a spiral structure or not?
How to do it?
To figure this out for yourself, you will produce a
map
of these clouds. For each cloud (there is
table below) you have two pieces of information: its direction in the plane of the Milky Way, given
1
as an angle, and its distance from us, given in thousands of light years. That is all you need to make
the map of these clouds and see if they outline a spiral structure. You will make the map using this
grid:
The bull’s eye marks our
position. Angle zero, towards the constellation Sagittarius is, you guessed
it, the direction towards the center of the Milky Way. Each concentric circle marks 5 thousand light
years in distance from us.
The first step is to
mark the location of the Galactic center
by dragging to the grid this symbol: Use
info from material (slides) if needed to complete this task. Are you sure you did it right?
2
Next, mark the position of each gas cloud from the table below. Galactic longitude is that angle
shown in the grid (0 to 360). To do this well, the trick is to estimate how far a cloud is from the grid
line when it is not exactly on it. Each wedge spans 30 degrees, so if the longitude is say 40, it is
third of the way from the grid line. If it is 45, it is half way, and so on. Same for distances, where
each circle is 5 units wide. IMPORTANT: Table continues in columns 3 and 4. I suggest you highlight
in the table the clouds you finished (bold, underline), so not to miss some or do it twice. To help
you mark, the longitudes are given in the increasing order. For full credit you need to mark all 58
clouds (there are exactly 58 dots). To make it easier to select and drag the dot you may want to
zoom in on this document.
Galactic
Longitude
Distance (1000
LY)
Galactic
Longitude
Distance (1000
LY)
10
14
174
11
13
15
190
3
19
15
192
13
21
16
197
10
23
7
202
3
25
20
222
13
26
10
233
6
28
22
234
13
29
22
243
16
46
15
251
18
46
13
281
8
50
18
283
11
52
23
285
16
55
23
286
21
54
20
291
23
58
22
292
26
79
27
295
7
88
21
307
6
92
9
312
25
106
18
315
23
108
17
324
21
110
4
328
6
114
14
331
18
123
12
336
17
134
14
339
14
138
5
339
16
144
11
342
16
151
14
345
6
160
13
352
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When all of the clouds are plotted, answer these questions.
3
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1.
How many arms can
you outline. (The
correct answer is in the
range between 2 and
8)
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2.
Which of the three
galaxies on page one
does Milky Way
resemble the most?
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3.
What is the typical
separation between
spiral arms in light
years? Estimate by eye,
no calculation is
needed
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4.
The spiral arms appear
to curve around a
specific point/location
in the Milky Way.
What is that?
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5.
Describe in your own
words why it is difficult
for astronomers to
map out the spiral
structure of the Milky
Way. (In general, not
what you found
challenging about this
exercise :)
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Extra credit:
Highlight several spiral arms. One suggested method is this: Use Insert, Shapes,
Freeform, then click along the arms. You need to close the shape by clicking on the starting point.
Finally, format the shape so that the fill color is transparent. Here is one example of such shape:
4
[Grading: Up to 4 points for clouds, 1 for center, 5 for questions]
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