In this assignment you are going to (1) plot the HR diagram for 3 different clusters, in three different ways, which will result in 9 graphs at the end of this. (2) Then you are going to do the same for the nearest stars to Earth. Stars in a given cluster and local stars to us are different populations: cluster stars formed at roughly the same time, while stars “in the field” (not in a cluster) like local stars can be of all different ages. Let’s see if their HR diagrams look different.
Attached you can see examples of the apparent magnitude vs B-V HR diagram for Pleiades, absolute magnitude vs B-V HR diagram for Hyades, and the Luminsity-Temperature HR diagram for Pleiades, so you can compare your versions and make sure you’re on track.
1. CLUSTER STARS.
We’re going to use the WEBDA astronomical database of stellar clusters. In the next couple of weeks we’ll be adding to the HR diagrams as we learn more about the physics that determines the patterns that emerge.
Procedure for the preparation of HR diagrams:
1) You will be plotting HR diagrams for the Pleiades and Hyades open clusters, plus one other of your choice. Here’s the link to the list of open clusters – pick your favorite!
2) Go to this page:
3) Under the search bar “Display the page of the cluster:”, enter the name of the first cluster (and hit enter!).
4) Scroll down: under “WEBDA content”, select “Data collection and references”.
5) Under the left panel headed “Photometry”, click on UBV (usually the top link). In the main panel you will see a list of reference numbers alongside a number of stars. These are papers where data has been published on that given number of stars in the cluster you selected. Make a note of the reference that contains the MOST number of stars.
6) Hit the back button twice to get back to the WEBDA page for the cluster, go back down under “WEBDA content”, and this time select “Available data”.
7) Under the left panel headed “Photometry”, click on UBV (usually the top link). This will bring up a table in the main window, which includes V (the apparent magnitude) versus the color index B-V; these are the important columns.
8) Cut and paste the table into a text file, and save it without formatting (as a .txt file).
9) Open Excel; Under the File drop-down menu, select import; select the .txt file containing the data. In the box that will open up, keep the option “Fixed Width” and hit Finish. The table will now appear with the individual entries in individual cells.
10) You’ll notice that most stars have multiple measurements from multiple different reference sources. We need to select only the reference that has the most stars. To do that, sort the data according to reference (select the reference column, and go to Excel’s Data tab, and select sort). That will allow you to select and delete all data except the dataset from the reference you noted in step 4.
11) With the remaining data, plot V (on the y-axis) versus B-V (on the x-axis). Make sure that the scale on the V axis runs from higher values at the bottom to lower values at the top, otherwise the plot will be upside down.
12) Repeat for the other 2 open clusters (you should have Pleiades, Hyades and one other).
13) Make sure the 3 HR-diagrams have the same ranges on their axes, so you can compare them directly.
14) Next, convert visual magnitude V into absolute magnitude by looking up the distance to the cluster, and using that in the distance modulus equation which you’ve used in past weeks. Plot
versus B-V on a separate plot. Make sure the scales are the same on both plots.
15) You need to now convert and B-V into log(T) and log(L/Lsun).
16) Plot log(T) (x-axis) versus log(L/Lsun) (y-axis), and orient the axes so that the plot looks similar to the version vs B-V.
17) Finally, on both sets of HR-diagrams, write the spectral types O-M underneath the horizontal axes.
2. FIELD STARS
You’re now going to repeat this procedure for the stars within 25pc of the Sun. Go to the database and make sure you select apparent magnitude, B-V color index and parallax. I would advise selecting B-V color index as the quantity sorted by, so that those sources without a measured B-V magnitude are easily filtered out. Then scroll down to the bottom and hit “search”. The results can be displayed in printer friendly format, and copied to a text file and imported into Excel just like the results above.
You will need to calculate the distance to all the stars from their parallax, and then convert the apparent magnitude into absolute magnitude using the distance modulus, all as learned in unit 1. Once that is done, you can follow the procedure in PART 1 to plot the 3 different forms of the HR diagram.