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.
Explore 6. 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, or
record the object ID and the five magnitudes of each star on a sheet of paper.
Launch the Object Explorer
Explore 7. Find the g-r, r-i, u-g, and i-z colors of each of
the 17 stars you examined in Explore 6.
Explore 8. Use Microsoft Excel or another graphing program to
make a color-color diagram of the 17 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 17 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
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 10. 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 9. Make another color-color diagram of the 17 stars
from Explore 6, with u-g on the x-axis and g-r on the y-axis.
Question 11. 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 12. 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.