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Thermal sources plot into a straight line on a color-color diagram. What do you get when you plot the observed colors of stars? This question is another way of asking the question you were asked in the “A Word of Warning” section: how close are stars to being perfect thermal sources?

You now know enough to find out for yourself.

Explore 5. Use SkyServer’s Object Explorer to find the stars with the following object IDs in the SDSS database. Look at the object’s magnitudes in SDSS’s five filters – they are the quantities u, g, r, i, and z in the second row next to the object’s image. Save the stars in your notebook. Use this SkyServer workbook to keep track of your data.

Object ID RA Dec

1237674649928663196
179.88431 -0.28125

1237674649928663243
179.94806 -0.2724

1237674649391792322
179.97106 -0.82002

1237648720693821693
179.8339 -0.55558

1237648720693821753
179.89941 -0.61227

1237648720693821562
179.97446 -0.55081

1237648720693821706
179.85499 -0.4708

1237648720693821769
179.91213 -0.50101

1237648720693821564
179.97887 -0.57861

1237674649928597663
179.789 -0.29908

1237648720693952659
180.20067 -0.57081

1237674649928728781
180.10892 -0.34719

1237674649391857908
180.11286 -0.77366

1237648720693952655
180.19612 -0.47336

1237674649391923286
180.16034 -0.72755

Explore 6. Find the g-r, r-i, u-g, and i-z colors of each of the 15 stars you examined in Explore 5.
Explore 7. Use Microsoft Excel or another graphing program to make a color-color diagram of the 15 stars from Explore 6, with g-r on the x-axis and r-i on the y-axis. The directions below will tell you how to use Microsoft Excel to make the diagram. To use another graphing program, you would follow similar steps.

Click on a box in the Excel spreadsheet. Enter the g-r color of one star from Explore 6. Hit the right arrow key, and the cursor will move to the box to the right of the first color. In this box, enter the r-i color of the same star. Click on the box below the first g-r box to move the cursor to the next line. Repeat these steps to enter the g-r and r-i colors of each of the 15 stars. You will end up with two columns of data, one for g-r color and one for r-i color.

When you have finished entering the data, click on the upper-left box and drag the mouse to highlight all boxes that contain data. Then click the chart wizard, the stylized bar graph in the tools bar at the top of the page. In the chart wizard dialog box, select “XY (scatter),” then click next. On the next screen, click next again. On the third screen, give your chart a title, then label the x-axis “g-r” in the Value X axis box, and the y-axis “r-i” in the Value Y axis box. Click Next, and then on the next screen, click Finish.

A graph of your data will appear on the same page. Click on the x-axis, and the axis will become highlighted. (If some other part of the graph is highlighted instead, click outside the graph and click the x-axis again.) Double-click the x-axis to bring up the “Format Axis” dialog box. Click the scale tab at the top of the window, then give your axis an appropriate scale. Double-click the y-axis, then change the y-axis scale so that you can see all seventeen data points clearly.

Question 8. Which end of the line in your graph corresponds to hotter stars? Which corresponds to cooler stars? How do you know?

Your graph shows that hotter stars tend to follow the trend of a straight line, but cooler stars diverge from this trend. This means that hotter stars can be thought of as thermal sources, but cooler stars can not.

Explore 8. Make another color-color diagram of the 15 stars from Explore 6, with u-g on the x-axis and g-r on the y-axis.
Question 9. Again, the hottest stars follow a linear trend, meaning they can be thought of as thermal sources. But in the u-g/g-r diagram, where does this trend begin to break down? What is the significance of this observation for thinking about real stars as thermal sources?
Question 10. If you know about stellar evolution – how stars change as time passes – you can answer this question. What is the significance of the flat line at the top of the u-g/g-r diagram? What types of stars are these?
Hint: what does it mean for g-r to be constant as u-g changes?

The last few exercises have shown you what the colors of stars can tell you. But what about other astronomical objects, like galaxies? Click Next to find out.