Astronomy 122 Third Homework Submission Template

1.  Stellar Parallax:  When you launch the simulator it should come up like this:

Then click:  Specify a catalog and chose 30 bright stars from the drop down menu and then also check, “specify a star”.  Your screen now looks like this:

You are to select two stars for this exercise – GL171.1 and GL 244.  Upon selecting a star, click on step 2 - Measure Stellar Parallax to get this measuring screen – every 6 months a measurement is made and plotted in what is called “Error Histogram”.  These errors are caused by making measurements through our atmosphere.   You need to wait until you have about 40 measurements until you have a reasonable certain average value in the error histogram. That average value is the stellar parallax angle for that star.  Once you know this angle the distance to the star (and hence its luminosity) can be determined.

Enter your answers to the questions below in the provided table (the response cell will expand with your typing)
Question:	Your Response	
a) What are the angles for GL 244 and GL 171.1		
b) Which one of these two stars has the more accurate determination of the parallax and why?		
c) Explain selecting only the brightest stars in the sky for parallax measurements would give you a biased sample		

2.   Explain why stars are unstable (that means their Radius changes) when they are not generating energy in their cores:

Your response here:	

3. For low mass stars, the relation between their luminosity (intrinsic energy output) and their mass is the following:

Which means luminosity is proportional to Mass to the 4th power.  The main sequence lifetime of a star is found by its Mass (fuel) divided by the rate of fuel burn (Luminosity); hence lifetime = M/L where M and L are measured in solar units.  A one solar mass star with one solar luminosity has M/L = 1/1 =1.   The main sequence lifetime of a one solar star is 10 billion years.  From that information answer the following and show all work.

Question:	Your Response	
a) What is the luminosity of a 5 solar mass star compared to a 1 solar mass star?		
b) What is M/L for a two solar mass star?		
c) What is M/L for a 0.5 solar mass star?		
d) What is the main sequence lifetime of a 10 solar mass star?		
e) What is the main sequence lifetime of a ½ solar mass star?		

4. In this exercise, you will be measuring the equivalent widths of absorption lines in 
real spectra of real stars using a simulation – make sure you access the video tutorial. If you don’t watch the tutorial, you will be lost – guaranteed.  Watch the video is the best way to learn how to measure the EW value, required for this assigment.  Let me say that again, watch the tutorial video!.   The number that represents the equivalent width is underneath the wavelength indicator in the reference level box.  

Note that this number/measurement is sensitive to how you define the start and end of the feature and where you place the reference mark.

Comparing the Hydrogen Line Strength of two different stars:

Question:	EW Value	Your Response	
a) From the list under step 1 choose the star type labeled A5-7V and measure the equivalent width (EW)  of the Hydrogen line at 4860 angstroms		Paste a screen shot of the A5-7V measurement here:	
b) Now make the same measurement of the star G8IV		Paste a screen shot or the G9IV measurement here:	
c) Explain why this measurement was more difficult for the G star compared to the A star			

5.  Stars are often born in stellar clusters.  You will need to download the Red and Blue images of a simulated stellar cluster (see downloads above).  Below are two simulated images of a stellar cluster; left one is taken in Blue light and right one is taken in Red light. (Though it is best to use the actual linked simulations rather than these still images)

                      BLUE                                 RED

There are 31 stars in this cluster.  Extremely blue stars would be much brighter in the blue frame than the red frame whereas extremely red stars would be much brighter in the red frame than the blue frame.  Use a 35 second exposure time for the red and blue images and answer the questions below:

Question:	Your Response	
a) Identify by star number, three blue stars		
b) Identify by star number, four red stars		
c) Identify by star number, five stars that appear of equal brightness in the two frames.