Spectral Types

This project teaches students about spectral types of stars. Students will learn that the spectral classification system is actually a temperature scale for stars. They will also learn what causes the absorption and emission lines in a star's spectrum, and why stars of different temperatures have different spectral lines. They will also have a brief look at stars that do not match the traditional spectral types.

If you are doing this project with younger students, you may wish to skip the section on energy levels. As long as they realize that the lines are caused by different elements and can identify the lines in spectra, they should be able to classify the stars.

For more information on spectral types of stars, read the About Astronomy: Stars section of SkyServer, or see one of the following references:

Kaler, James, Stars and Their Spectra, 1997

Spectra, a web page by James Kaler

Project Goals

By the end of the project, students should be able to:

• understand that each star has a unique spectrum, with a series of absorption and emission lines
• understand that stars emit thermal radiation with a peak wavelength that depends on the star's temperature
• explain that stellar absorption lines are caused by electrons in the outer layers of a star's atmosphere absorbing photons from the star
• know that the pattern of absorption lines depends on the temperature of the star
• classify the spectrum of a star from its absorption lines
• find the temperature of a star from its thermal radiation curve if the peak is visible
• identify which types of stars are most common, and understand that some types of stars are too faint observe, even with large telescopes

Background Knowledge

Students should have a basic understanding of the nature of light before undertaking this project. They should know that light is a wave and that different wavelengths correspond to different colors. Students also need to be able of read and understand graphs of spectra.

The only mathematical calculation is relatively simple - finding the temperature of a star from its thermal radiation curve.

Project Structure

The first section lets students try to classify stars based on their spectra. The students should start learning to look for patterns in spectra during this section. After developing their own classification system for stars, they will discuss their system with another group and try to strengthen the system. It is a good idea to set a firm time limit, or students will take a long time on this section. Estimated time: 30 - 40 minutes

Next, students will learn a little about the energy levels of atoms, and how electrons jumping between energy levels can absorb and emit light. Students should make the connection between atoms absorbing and emitting light and the peaks and valleys they see in stellar spectra. Estimated time: 20 minutes

Then, the students will learn that different elements show absorption lines at different temperatures. They will learn how to use these lines to classify stars, and how to use the lines to estimate the temperatures of stars. Estimated time: 20 minutes

Students will have an opportunity to practice classifying stars. Finally, they will compile all their data in an attempt to find out which types of stars are most common, and which types of stars are least common. Estimated time: 45 minutes

The information in this unit is closely related to the Colors unit, and it comes up again in the H-R diagram unit.

Questions and Exercises

Questions are designed to get students thinking about the way scientists work. Exercises are designed to get students to explore SkyServer data to solve problems. For answers to all questions, click here. To see the answers, you must have the Adobe Acrobat PDF viewer installed on your computer. Acrobat is available for free download from Adobe's web site.

Students should be evaluated based on their written answers to the questions and exercises. You may use our sample scoring rubric or develop your own. If you use our scoring rubric, print out a copy for each student and attach it when you return his or her work.

For specific information on any part of the project, click Next.