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ASTR1040 Spring, 2005
Due date: Tuesday, April 5
1.The following data were collected on the apparent magnitude (m_v) of a Cepheid variable:
Time (days) m_v
0.0 +28.47
4.2 +28.23
5.4 +28.20
11.8 +27.72
14.8+27.41
15.9+27.39
18.8+27.05
20.4+27.02
24.2+27.35
26.6+27.56
32.8+28.03
38.1+28.30
39.7+28.38
40.9+28.42
42.0+28.45
43.9 +28.48
48.2 +28.28
54.2 +27.87
56.5 +27.66
57.6+27.54
58.7+27.45
62.2+27.14
65.8+27.03
63.9+27.00
68.0+27.27
74.4+27.84
78.0+28.06
79.1+28.14
83.2+28.33
88.9+28.51
(My apologies for the garbled format here; I have not yet learned how to set up proper tables in the html program I use. You should note that each entry has two values, one being the time of the observation in days from zero; and the other being the observed apparent magnitude of the star at that time. Thus the third entry, just to pick one as an example, is for time = 5.4 days and apparent magnitude m_v = +28.20.)
Using these data, find the distance to the Cepheid. To do this, you will need to graph the data listed above, in order to find the period of pulsation and the average apparent magnitude (estimate this by finding the mean between the minimum and maximum values, derived from your graph). Then find the average absolute magnitude, using the period-luminosity relation below (graph provided with printed assignment only), and from that, find the distance using the distance modulus.
How many light years away is this Cepheid? Is it within our own galaxy? Comment on the utility of Cepheids as tools for probing the size scale of the universe.
2. Suppose the rotation curve for a spiral galaxy looks like this (graph provided with printed assignment only):
(a) What is the mass of the galaxy?
(b) What assumptions were adopted in the technique you used?
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