Ask students what they see when they look at the stars. At least some of their answers should indicate that stars have different colors. Ask, “do you ever wonder why stars have different colors?” Tell them that in this project, they will learn why.
Colors of Stars in the SDSS
The key to doing Explore 1 is using the Navigation Tool. Students should choose a random stripe and mosaic, then click randomly at different points on the mosaic to look at different zoom views. They should search freely through the stars in the Zoom window to find stars of different colors. Remind them to make sure they choose stars and not galaxies. The top of the browser window will show the object type and its magnitudes in the five wavelengths (u,g,r,i,z). Students should make a note of the star’s color on a sheet of paper, then click “Add to notes” to save the star in their notebook. A red check will appear next to “Add to notes” to show that the star’s information has been saved.
Sometimes, a bright star will be mis-classified as a galaxy. This misclassification happens because the software SDSS uses to classify objects classifies pointlike light sources as stars and extended light sources as galaxies. When the CCD camera sees a very bright star, some of the star’s light spills over into neighboring pixels, making the pointlike star look like an extended light source. You can recognize a bright star by diffraction spikes – bright crosses centered on the star – that occur as light bends inside the SDSS telescope.
In Explore 2, students should copy the information they saved in their online notebooks into a spreadsheet program. They should use their handwritten notes to enter each star’s color. They should then use the sort feature (under the Data menu in Excel) to look for patterns in the data. They should sort first by magnitudes (u,g,r,i,z), then by differences in magnitudes. When they see the colors begin to line up in some order (when stars of similar colors are in similar places on the list), they should make a note of how they sorted the data.
Ideally, they should find that the colors are sorted (blue stars on one end and red on the other) when the data are sorted by differences in magnitudes. Remind students that they shouldn’t expect every star to fit the pattern perfectly; they should expect to see only a general trend. If they are unable to see any trend at all, suggest that they return to the Navigation Tool and collect more data.
The Definition of Color
This section builds on the work students did in the last section, offering a concise mathematical definition for color. Astronomers use this definition when they discuss color in their research.
Color is defined in terms of stellar magnitudes. Students may be interested in learning the history of the magnitude system, which can seem arbitrary and confusing. The “Did You Know?” in this section discusses the magnitude system’s history.
Remind students that magnitude decreases with increasing stellar brightness. So if a star emits more red-wavelength light than green-wavelength light, its red magnitude r will be less than its green magnitude g; therefore, its g-r color will be positive.
This section includes a “Try This” activity designed to give students some intuition about magnitudes. Students shine two flashlights toward a specified point: one flashlight at 1 meter and the other at 1.58 meters. When they look at the two flashlights, the nearer one will appear about one magnitude brighter. These distances were calculated using the inverse-square law of light along with the definition of magnitude. The activity may work better with light bulbs, which are omnidirectional light sources. If you use light bulbs, make sure students are supervised so they do not burn themselves.
Use students’ intuitions about the word “filter” to express the concept of telescope filters. A filter is something that collects only what it is designed to collect. A telescope filter blocks all light except for light with the specific wavelength it was designed to see; for example, red telescope filters collect only red-wavelength light. Once astronomers have used a filter to collect light at a certain wavelength, they can calculate the star’s magnitude in that wavelength; that is, the magnitude that star appears when seen through that filter. If your school has a theater, you may borrow some colored gels from the theatrical lights to demonstrate what filters do.
If students are confused about why color should be defined by the difference in magnitudes, appeal to their intuitions. If a star looks red, it must emit more red light than green; therefore, the difference between the amount of red light it emits and the amount of green light it emits should be positive. The reason color depends on the difference between magnitudes and not the ratio is that magnitude is a logarithmic representation of brightness, so differences in magnitude correspond to ratios in brightness.
Thermal Radiation, Temperature and Observed Spectra
This section introduces the concept of thermal radiation. Some students are probably familiar with night vision goggles, which detect infrared thermal radiation given off by objects.
Students will use the Java applet to explore the behavior of an object’s thermal radiation curve as the temperature of the object changes. Students should notice that the peak wavelength decreases as the temperature increases. This may seem counterintuitive to some students. Short wavelength radiation has higher energy, so it may be useful to mention that as the temperature increases, the energy of the peak wavelength increases.
You should familiarize yourself with the applet before your students do this section. The applet automatically resizes the y-axis when the temperature changes. You may wish to point this out so that students realize high-temperature objects give off more total radiation than low-temperature objects, in addition to radiation with higher peak wavelengths.
Color and Temperature relates the concept of thermal radiation to their everyday experiences with hot plates. Students should realize they can feel the thermal radiation as heat before the hot plate starts glowing red hot.
Students then look at the observed spectra of stars to see that stars are not perfect thermal sources. Their thermal radiation curves have various peaks and valleys due to the emission and absorption of radiation by elements in the stars’ atmospheres. Even with these peaks and valleys, students will frequently be able to see the underlying shape of thermal radiation curves in the spectra of stars.
Color-Color Diagrams and Thermal Sources
This section teaches students how to make and interpret a color-color diagram, a common tool of astronomers. Color-color diagrams can be hard to explain in words, but students can click on the image to see what the axes on a color-color diagram are. Have students study the diagram, and remind them that magnitudes decrease for brighter stars. When students are comfortable with what color-color diagrams are, move on to Explore 5.
Explore 5 lets students make a color-color diagram for themselves, and then use it to learn which stars can best be thought of as thermal sources. To find the given stars, students should click the links, or use the “Find by ObjID” feature of the Object Explorer. A new window will open for the Object Explorer, and students should read the magnitudes u,g,r,i,z from the second row next to the object’s image.
The project includes a link to SkyServer’s How-To tutorial on Microsoft Excel. If your students are more familiar with another graphing program, use that program instead. You may wish to pause here to give a quick tutorial of the program. Question 3 asks students which end of the color-color diagram corresponds to hotter stars. If students get stuck, ask them to think about only one axis at a time.
Students will see a straight line trend that will deviate for the coolest stars. Cool stars have atmospheres that absorb a lot of gas, so they show a lot of stellar absorption lines in their spectra. Because their spectra do not look like thermal radiation curves, these stars do not behave as thermal sources.
This section introduces the Color project’s Research Challenge. The research challenge should not be done in the classroom, because it is a completely open-ended exercise. Students think of an astronomical research question that can be answered by studying star colors. They develop their question, choose objects from the SDSS database to examine, and perform all analyses needed to answer the question. Encourage students to complete this exercise on their own, for fun. You may wish to offer extra credit to students who do it. If they are interested in doing the exercise, you should discuss their research questions and approaches with them outside of class.
The question asked should be a fairly simple question that can be answered by examining 20-40 objects using a straightforward analysis. Most likely, students will either make color-color diagrams or analyze peak wavelengths of spectra to answer their question. Be sure that they use color somehow in finding their answer. The Research Challenge lists a few suggested questions that students can answer.
The research projects in the Research Challenge can easily be extended into Science Fair projects if students are interested. We encourage students to use SDSS data in Science Fair projects.
To see how the Color project correlates with national science and math learning goals, click Next.