Lesson Plan - Colors of Stars

Suggestions to Instructors

When being introduced to the blackbody curve, students can often interpret the graph as being luminosity with respect to time, as opposed to luminosity with respect to wavelength.

Also, students have a misconception that as a object gets hotter, it glows whiter, and that the hottest objects appear white to the human eye, as opposed to blue.

The blackbody viewer is designed to allow students to simultaneously see a graph of the spectrum, the spectrum as it would be viewed through a diffraction grating, and the apparent color of a heated object.

If possible, combine this with a carbon arc lamp connected to a variable power source, so that you can gradually increase the voltage, and have the students see what happens to the brightness and color of the arc lamp. If the students view this through diffraction gratings, they will see the visible part of the electromagnetic spectrum, which can be compared to the spectrum shown in the applet. A typical carbon arc lamp will not be able to heat up to the point that you will see a blue light, but stars in the sky are hot enough to produce an apparently blue object.

Make the point that the curve in the applet is showing the exact same thing as the colored spectrum, the only difference is that one is a plot, and the other is the result of different types of light bending more through a refraction grating.

If you have charged gas lights available as well, and can view the light produced through a spectral grating, contrast the sharp line radiation produced by an excited gas with the smooth continuum radiation produced by a heated solid.

Standards

The exploration meets the following National Standards:  Science Content Standards: 9-12

• CONTENT STANDARD A:
• Abilities necessary to do scientific inquiry
• Understandings about scientific inquiry
• Fundamental abilities and concepts that underlie this standard include:
  • Use technology and mathematics to improve investigations and communications.
  • Formulate and revise scientific explanations and models using logic and evidence.
  • Recognize and analyze alternative explanations and models.
  • Communicate and defend a scientific argument.
• CONTENT STANDARD B:
• Physical Science Standards
  • Structure and properties of matter
  • interactions of energy and matter
• Earth and Space Science Standards
  • Origin and evolution of the universe
  • Origin and evolution of the earth system
• History of Science Standards
  • Nature of scientific knowledge

Solutions

1. Describe what happens to the color of a stove burner or a coal as it gets hotter.
   It will begin to glow red.
2. Based on the knowledge of the stars brightness and color, what would you assume about the stars relative temperature? List the stars from coolest to hottest.
   As this is a question to allow students to confront misconceptions, the important thing is not for the students to get this question correct, but to write down what they think before actually attempting the exercise. The most common case is for students to think that the "white hot" object will be hottest, but results will vary from student to student.
3. Using the applet, determine what range of temperatures would give you a red light, what range of temperatures would give you a white light, and what range of temperatures would give you a blue light.
   Stars with a temperature less than about 4000K appear red. Stars with a temperature between 4000 and 10000K will appear mostly white, with possibly a faint shade of red or yellow at lower temperatures or a faint shade of blue at higher temperatures. Stars with a temperature over 10000K will appear bluish.
4. Apply this to the results you have from question 1. Is you list correct? If not, put the stars in the correct order from coolest to hottest.
   Red, White, Blue.
5. With the three stars from Orion that were listed, the red and light blue stars were very bright, but the deep blue star was somewhat dim. If the deep blue star is hotter than the red star, what are some reasons the star might appear less bright?
   The star may be further away, or smaller in size, or obscured in some way. The red star in Orion is in fact very close to us, and very large. It is the only star other than our own sun for which we have actually been able to see it in a telescope as a sphere (and only in the most powerful telescopes at that). All others are so small and far away that we can only see a pinpoint of light, no matter how much we "zoom in" with our telescopes.