Lesson Plan


By the end of this project, the student will be able to:

  • Identify key features of spectra, including the continuum peak and absorption lines
  • Explain how atomic energy level transitions explain spectral absorption lines
  • Classify stars by Hα line strength or continuum temperature
  • Explain the connection between hydrogen line strength and stellar temperature


Students should have worked with spectra before. They should be able to recognize the general shape of a spectrum, and should be able to tell the difference between noise and absorption lines.


  • Absorption line – A “valley” that appears in a spectrum due to light being absorbed by energy level transitions in atoms
  • Ångstrom – A unit of length measurement used for spectral wavelengths; one Ångstrom is one ten-billionth of a meter (1 Å = 10-10 m)
  • Balmer series – The series of absorption lines resulting from transitions down into the second energy level (n=2) in the hydrogen atom
  • Continuum – The smooth curve of a spectrum
  • Continuum peak wavelength – The wavelength at which the spectral continuum is greatest; it appears as the top of a broad hill in the spectrum
  • Emission line – A “peak” that appears in a spectrum due to light being emitted from energy level transitions in atoms
  • Energy level – An amount of energy that an electron can have in an atom
  • Hydrogen alpha (Hα) line – An absorption line associated with the transition from the third energy level (n=3) to the second energy level (n=2) in the hydrogen atom
  • Noise – Random variations in a spectrum that are not associated with the continuum or emission or absorption lines
  • Spectrum – A graph of the amount of light given off by an object at different wavelengths
  • Thermal radiation – Radiation given off by hot objects, such as stars


Students should work in groups of two or three. If you want to do the activity online, each group will need a computer with a web browser. If you want to do the activity in a classroom setting without Internet access, each group will need the following documents, available as a PDF document (requires Adobe Reader), or a Microsoft Word document.

Student worksheet

Stellar spectra

Filled example of counting grid squares

Preparation Time

The only preparation time required is to set up computers, provide the link to the students, and to duplicate the worksheet and the stellar spectra handout.

Also, you are strongly advised to try the activity beforehand, and practice with the Quick Look Tool.

Classroom Time

In a college lab setting, the full activity can be done during one two-hour lab period. If pressed for time, the first activity (classifying by spectral class) can be assigned as a pre-lab.

In a K-12 classroom setting, the full activity can be done in three 45-minute class periods.

Follow-up activities

The Research Challenges listed at the end of the student project give students ideas for further work after the main project is complete. These follow-up projects make good ideas for an end-of-term final project or a Science Fair project.

Few or no computer classrooms

This activity can be done without access to computers by printing out the spectra and student worksheet, and answering all questions by looking at the spectra.

Home schoolers

The project can be done by an individual rather than a group. It would be best if a student working alone could exchange his or her results with another student to check the work.