Distance to the Center of the Milky Way
Adapted from Learning Astronomy by Doing Astronomy by Ana Larson
Summary
In this exercise, you will use the locations of globular clusters in the halo to estimate the distance of the Sun from the center of the Milky Way.
Background and Theory
In the not-too-distant past, astronomers though that the Sun was at the center of our galaxy, the Milky Way. Observations and determinations of distances were hampered by the lack of knowledge of interstellar dust, which blocks much of the starlight from distant parts of the galaxy (including the galactic center). It was not until the distances to globular clusters were determined using the RR Lyrae stars that a more accurate picture of the size and shape of our galaxy was constructed. By determining the distribution of the globular clusters, Harlow Shapley was able to determine the diameter of the galaxy, and the distanc to the galactic center.
Procedure
- Using the polar graph in Figure 1, plot the galactic longitude versus distance for the globular clusters in Table 1. The Sun is at the center of this polar graph. Note that the distance given is not the actual distance, since we have projected a 3-dimensional space onto a 2-dimensional piece of paper. The actual distances are greater than those given here.
- Estimate the center of the distribution of globular clusters, and mark it on the graph. Describe how you defined the center of the distribution.
- Determine the distance from the Sun to the center of the distribution.
- Determine the direction to the center of the distribution. This is the direction to the center of the galaxy.
- In which constellation does the center of the galaxy lie?
- At what time of year is this constellation most conspicuous? Hint: check a planisphere, or the textbook.
- Why is the Milky Way Galaxy more spectacular in the summer than in the winter? (ignore weather conditions!)
- Describe the two dimensional space distribution of the globular clusters.
- How do we know the Sun is not at the center of this distribution?
- During their long orbit around the center of the Milky Way Galaxy, each globular cluster will cross through the plane of the disk. Why do we find most globular clusters far out in the halo? (Hint: Do Kepler's laws apply to globular clusters?)
Figure 1: Polar Plot of the Distribution of Globular Clusters
Table 1: Globular Cluster Data
NGC # |
Gal.
Long. |
Projected
Distance
(kpc) |
|
NGC # |
Gal.
Long. |
Projected
Distance
(kpc) |
|
NGC # |
Gal.
Long. |
Projected
Distance
(kpc) |
|
NGC # |
Gal.
Long.
|
Projected
Distance
(kpc) |
104 |
306 |
3.5 |
|
6273 |
357 |
7 |
|
288 |
147 |
0.3 |
|
6284 |
358 |
16.1 |
362 |
302 |
6.6 |
|
6287 |
0 |
16.6 |
|
1904 |
228 |
14.4 |
|
6293 |
357 |
9.7 |
2808 |
283 |
8.9 |
|
6333 |
5 |
12.6 |
|
Pal 4 |
202 |
30.9 |
|
6341 |
68 |
6.5 |
4147 |
251 |
4.2 |
|
6356 |
7 |
18.8 |
|
4590 |
299 |
11.2 |
|
6366 |
18 |
16.7 |
5024 |
333 |
3.4 |
|
6397 |
339 |
2.8 |
|
5053 |
335 |
3.1 |
|
6402 |
21 |
14.1 |
5139 |
309 |
5 |
|
6535 |
27 |
15.3 |
|
5272 |
42 |
2.2 |
|
6656 |
9 |
3 |
5634 |
342 |
17.6 |
|
6712 |
27 |
5.7 |
|
5694 |
331 |
27.4 |
|
6717 |
13 |
14.4 |
Pal 5 |
1 |
24.8 |
|
6723 |
0 |
7 |
|
5897 |
343 |
12.6 |
|
6752 |
337 |
4.8 |
5904 |
4 |
5.5 |
|
6760 |
36 |
8.4 |
|
6093 |
353 |
11.9 |
|
6779 |
62 |
10.4 |
6121 |
351 |
4.1 |
|
Pal 10 |
53 |
8.3 |
|
6541 |
349 |
3.9 |
|
6809 |
9 |
5.5 |
O 1276 |
22 |
25 |
|
Pal 11 |
32 |
27.2 |
|
6626 |
7 |
4.8 |
|
6838 |
56 |
2.6 |
6638 |
8 |
15.1 |
|
6864 |
20 |
31.5 |
|
6144 |
352 |
16.3 |
|
6934 |
52 |
17.3 |
6171 |
3 |
15.7 |
|
6981 |
35 |
17.7 |
|
6205 |
59 |
4.8 |
|
7078 |
65 |
9.4 |
6218 |
15 |
6.7 |
|
7089 |
54 |
9.9 |
|
6229 |
73 |
18.9 |
|
7099 |
27 |
9.1 |
6235 |
359 |
18.9 |
|
Pal 12 |
31 |
25.4 |
|
6254 |
15 |
5.7 |
|
7492 |
53 |
15.8 |
6266 |
353 |
11.6 |
|
|
|
|