Jason Kendall, William Paterson University, Department of Physics HR Diagram of Globular Cluster Messier 80 Using Hubble Space Telescope Data Background Purpose: HR Diagrams are central to understanding stellar evolution. Here, we ll take real Hubble Space Telescope data, and make an HR diagram out of it. Method: Download the data to your computer. Import it into Excel, and make a color-magnitude diagram, which is the same as an HR Diagram. You ll need to put your answers in a separate Word or PDF document and submit via Blackboard. 1 Basics 1. What is a globular cluster? 2. What kinds of stars are in them? 3. How old are they? 4. Is there star formation going on? 5. Google some pictures of globular clusters and describe their general appearance. 1
2 Interpreting the images of Messier 80 Go to http://hla.stsci.edu/. Click "Enter Site." Search for M80. Then do an "Advanced Search". Check off only the WFPC2 instrument. Search for Proposal ID 11233. Two "visits" come up, and eight results appear. Six of these results are not relevant to this lab. You only want to use "visit 6" for this lab. Locate the F450W and F814W data. Ignore the "F814W/F450W" and "detection" spectral elements. Click on "Display" for the "Visit 6" F450W data. 1. When the image comes up in a new window, zoom in and out, pan around and describe what you see. (hint: none of these answers are one or two words...) 2. Now click on the Advanced Contrast Controls. Play with light/dark and describe what you see as you make the changes between light and dark. 3. In the Image Viewer, check the box for DAOPHOT. All the circled stars are data. How many stars are circled when you check DAOPHOT on the F450W data? 4. Why, perhaps, were some stars not be circled and kept as data? 5. Now look do the same with the display image for the F814W data. How many stars are in the DAOPHOT dataset for the F814W image? 6. Between the two images, which one seems to show the stars being brighter using the default settings in the image viewer? 7. Compare and contrast your two images to this image of Messier 80: http://hubblesite.org/newscenter/archive/releases/1999/26/image/a/format/web_print/ 8. Roughly where in M80 are you looking with the Hubble Data? Are you looking at the center, around the center, or far on the outskirts? 9. Justify your answer above. 10. With that in mind, why did the team doing the observations of Visit 6 look where they did? (Hint: just think about the circled data that you see in relation to the fancy image.) 2
3 Astronomical Filters Now we want to understand the filters that were used to create the images you just looked at. Look at this web page: http://svo2.cab.inta-csic.es/theory/fps/index.php?mode=browse&gname=hst&gname2=wfpc2 Look at the filter plots. The X-axis is wavelength in Ångstroms, and the Y-axis is transmission percentage. The top of the Y-Axis is "1", which is 100% transmission. The bottom is "0" which is 0% transmission. 1. What is the range of transmission wavelengths of the F450W filter on the WFPC2? 2. What is the range of transmission wavelengths of the F814W filter on the WFPC2? 3. Given that visible light ranges between 4000 and 7000 Ångstroms, what kind of light do the two filters appear to allow to pass? Indicate all that apply. Radio Microwave Infrared Red Visible Green Visible Blue Visible Ultraviolet X-ray Gamma Ray F450W F814W 4. Is the F450W a "red", "green" or "blue" filter, meaning what color would things be if you looked through the filter at something? 5. Is the F814W a "red", "green" or "blue" filter? 6. Is there any overlap to the filters? 7. How many other filters does the Hubble Space Telescope use? 8. Name three filters that do not overlap with each of F450W and F814W. 9. What is the general purpose of an astronomical filter? 3
4 Magnitudes and color-temperature Suppose you have four stars in the same cluster measured for their brightnesses using the magnitude system of measurement in two filters; a "red" filter and a "blue". Here are their apparent magnitudes: Star A: blue magnitude = 21.7, red magnitude = 20.4 Star B: blue magnitude = 22.3, red magnitude = 20.2 Star C: blue magnitude = 20.1, red magnitude = 21.4 Star D: blue magnitude = 23.0, red magnitude = 19.6 For the exercise below, we define color as "blue magnitude minus the red magnitude". 1. Which star (A/B/C/D) is brightest in the blue? 2. Which star (A/B/C/D) is brightest in the red? 3. What are the "blue - red" colors of each star A,B,C,D? 4. Which star (A/B/C/D) is hottest? 5. Which star (A/B/C/D) is coolest? 6. How does the numeric color above relate to temperature? 7. How does this relate to blackbodies? 4
5 Download and reduce the Hubble data Go to http://hla.stsci.edu/ Click "Enter Site." Search for M80. Then do an "Advanced Search". Check off only the WFPC2 instrument. Search for Proposal ID 11233. Two "visits" come up, and eight results appear. Six of these results are not relevant to this lab. You only want to use "visit 6" for this lab. Locate the F450W and F814W data. Ignore the "F814W/F450W" and "detection" spectral elements. Now to download the Hubble Data to your computer. Put in the cart-basket the DAOphot data of WFPC2 for F450W, "visit 6", and not the FITS or SEXphot data. Just get the DAOphot data. Next, put in the cart-basket the DAOphot data of WFPC2 for F814W, not the FITS or SEXphot data. Just get the DAOphot data. Go to the checkout and download that data to your computer. It will come down in a ".cat" file format. Rename the files to ".txt" at the end. Now to reduce your data. You ll just be doing a small part of it for now. First, open the F450W data in Excel or a similar spreadsheet tool. If you don t have Excel, then you can download OpenOffice for free from www.openoffice.org. It can do all those things and more. Now open the F814W data in Excel. On both data sets, we want to delete all the columns except "ID" and "TotMag". (i.e. the identification number of the star and its magnitude.) These are columns 5 and 13. Be VERY careful how you separate them into columns. This is where it can EASILY go wrong. The columns are fixed-width delimited, and contain lots of spaces. Probably the best way to import it to Excel is to open it with Word or something like that. Then copy and paste the whole thing into the Excel page. It ll all be one column. Then you do "Text to Columns" and use "fixed width", not delimited. Excel should detect the proper widths. You may just delete the header rows, and the extraneous columns. Next, sort the data by the ID number. The ID numbers run from 1 to well over 16000. Next, is the really big job, and why it s extra credit. Find all the stars that have magnitudes in BOTH filters. To get the data you need, combine the two spreadsheets into one spreadsheet. Now, REMOVE any star that only has a magnitude in only one filter. KEEP only the stars with magnitude measurements in both filters. This will reduce it down from 6000 stars. You should now have lined them all up such that you have a star ID and its two magnitudes. And you will have thrown away any data which didn t have both magnitude measurements. Then create a third column for the colors of these stars. To do this, create a new column in your spreadsheet containing a difference between the two magnitudes for each star. Let s difference all the values like this: Color = F450W - F814W Now plot the data using Excel s graph feature (or use any other plotter) with Color on the "X" axis, and the F814W Magnitude is on the "Y" axis. It helps a lot to simply copy the F814W data into a column to the right of the "color". Make sure that you make the points of your plot very small, so you can see the shape of the diagram. Make sure that you arrange it so that "brightest" is up. You ll need to remember the definitions of magnitude to make sure it s correct. Look at the example HR diagram at the bottom of this document. This is what about what your HR Diagram needs to look like when you are done. Print out your plot when you re finished. 5
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6 Collecting samples of data from the HR Diagram 1. How many stars are in this H-R diagram? 2. What s the numeric range of colors of the stars on this H-R diagram? 3. What s the numeric range of magnitudes of the stars on this H-R diagram? 4. What s the magnitude of the brightest stars on this H-R diagram? 5. What s the magnitude of the dimmest stars on this H-R diagram? 6. What are the color and magnitude of the hottest star on this H-R diagram? 7. What are the color and magnitude of the coolest star on this H-R diagram? 8. What would make you call a star "blue" on this diagram? 9. Are the stars on the plot mostly "red" or "blue" or in between? 10. Are there more giants or main sequence stars in this H-R diagram? 11. What is the greatest difference in magnitude between a giant star and a main sequence star with the same numeric color on your HR Diagram? 12. How many times more luminous is this giant star than this main sequence star? 13. How many times larger in radius is this giant star than this main sequence star? 7
7 Making Sense of the Data. 1. What is the color and magnitude of the turnoff point of the cluster? 2. What is the significance of the turnoff point in this, or any other, HR diagram? 3. Look up an accurate age of the globular cluster Messier 80. 4. If the Sun had instead been born in this cluster and was as old as this cluster, in which of these groups would it appear right now: Protostar/Pre-Main-Sequence? Main Sequence? Giant Branch? Horizontal Branch? Blue Stragglers? White Dwarfs? 5. Justify your answer above. 6. Given the age of this globular cluster, roughly how many of the main sequence stars on this plot are about the same mass as the Sun: 0? 10? 100? 1000? 7. Look up the mass-age relationship of stars on the Main Sequence in your textbook. What spectral types of stars CANNOT be in this main sequence on your HR Diagram that you just made? 8. Justify your answer above. 9. Draw on the plot the rest of the Main Sequence. 10. Mark on the plot to indicate the Sun s position on the HR diagram roughly where it would have been on the Main sequence, if it had beet part of this cluster. 11. Mark on the plot to indicate where the Sun would be now if it was born in this cluster. 12. Imagine that the planet Earth survived being around this fictitious "Sun" all the way from beginning to end. Some people claim that life cannot exist on that planet. Do you agree or disagree. Why or why not? 13. Imagine you looked at the chemical composition of this fake Sun and compared it to our current Sun. Would they be the same? Why or Why not? 8