### ASTRONOMY 3303   HW#1     due Wed Aug 30, 2017

We assume that you also will have read Chapter 1 and Appendices A and B of the textbook "Extragalactic Astronomy and Cosmology" by Peter Schneider. After that, you should start reading Chapter 2.

Please understand that we are looking for rigor and clarity in your answers. Be sure to (1) cite any sources you use, (2) show your work and (3) define any terms or symbols that you use. If possible, please type your work (as much as possible), adding symbols/equations/calculations by hand as necessary. Thank you.

a.   Suppose a beam of radiation of incident intensity Iν encounters and enters a cloud of optical depth τν. If the total optical depth of the cloud is equal to 1, what is the intensity of the radiation Iν,e that emerges from the cloud on the other side, assuming that ther is no emission within the cloud itself?

b.   What is the absolute magnitude of a star of apparent visual magnitude 9 (assuming there is no impact of dust) that lies at a distance of 100 pc?

c.   What is the approximate spectral type of the star in part 1b, if it is a Main Sequence star? Be sure you tell us precisely and clearly how you arrived at your answer.

d.   Astronomers sometimes say that high mass stars are "gas guzzlers"? What's the reasoning behind that analogy?

e.   Suppose two stars at the same distance from us have the same surface temperature. You observe one star to be 100 times brighter than the other star. What can you conclude about their relative size? Be sure you can explain how you got your answer.

f.   Compute the solid angle of the full sky in square degrees. Do not just quote the answer but rather show how you computed it.

2:   Explore TOPCAT

a.   Learn how to use TOPCAT, the Tool for OPerations on Catalogues And Tables. Try to download and install TOPCAT on your own laptop/computer. Can you install it successfully? If you are unable to do this, talk with us ASAP, so we can figure out what your problem might be.

b.   Make a scatter plot of the two columns in this simple CSV ("Comma Separated Variables") file. What do you find? Include a copy of the output with your homework.   Hint: be sure you set the "format" to "CSV" before you try to load the table.

c.   Here is a CSV compilation of the basic properties of a large sample of nearby stars. Note: this is a pretty large file (10MB); it may take a while to load in your browser; then you need to save it.)

Use TOPCAT to plot the distribution of stars on the celestial sphere (spherical plot; export the front face of it to include in your assignment).   Hint: you need to change the units from R.A. in hours to R.A. in degrees. How can you do that?

d.   Using the same file, plot the distance (Distance, in parsecs) on the X axis and absolute magnitude (AbsMag, in magnitudes) on the Y axis. Set the y-axis so that the luminosity increases from bottom to top. These are real data, and some of them aren't "pretty" (are bogus or missing), so you need to set the range on the axes to exclude far outliers. Set the x-scale from 0 to 1000 and the y-scale from 15 to -8. Why is it conventional to flip the y-axis? How can you explain why are there (almost) no points in the bottom right part of the plot?

Hang on to this file; we will use it again next week!

3:   The Cosmic Microwave Background (a first look at the CMB)

Read the assigned sections of the first chapter of the textbook as well as App. A and B. Then check sites on-line to familiarize yourself with the CMB; we will discuss it in more detail later. As you answer these questions, be sure to record complete references to the sources of any information you use.

Take a look at the Astronomy Picture of the Day APOD for 2005 Sept 25 which shows the Cosmic Background Radiation as measured by WMAP (shown just to the right). Then take a look at the 2013 version (from 2013 March 25), obtained by the Planck satellite (shown to the far right).

a.   The image shown as Fig. 1.21 of the 2015 textbook (1.17 of earlier version) was taken by COBE, whereas the images displayed on the APOD site come from WMAP and Planck respectively. What are COBE, WMAP and Planck and in what ways are they different?

b.   The Planck APOD image is supposed to show a "high-resolution map of microwave light emitted only 380,000 years after the Big Bang". What do the different colors in the image represent? What is the amplitude (in both temperature units and in angular units) of the fluctuations? Be sure to record for yourself what the image tells us about the early universe.

c.   What does the Planck map reveal that is different from the WMAP one?