A Color-Color Diagram for SDSS Stars
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.
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 |
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.
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.
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.