Authentic Lessons for 21st Century Learning

Classifying Stars With Spectra

Light, Stars, and Blackbody Radiation

Lindsey Link, Matthew Peck | Published: March 17th, 2022 by K20 Center

  • Grade Level Grade Level 8th, 9th, 10th, 11th, 12th
  • Subject Subject Science
  • Course Course Chemistry, Earth Science, Physical Science, Physics
  • Time Frame Time Frame 2-4 class period(s)
  • Duration More 150 minutes


Students will sort 14 star spectra and create a star classification system to develop science classification skills. Through analysis of figures that describe the OBAFGKM classification system and temperature effects on element absorption in stars, students will learn that star spectra can be used to find a star's surface temperature. A summative reading describes OBAFGKM star types that are more likely to harbor planets that could support complex life, reinforcing the connection between star spectra, temperature, and science's search for habitable planets. This lesson follows "Emission Spectra of Excited Gasses," in which students learn about light energetics and spectra.

Essential Question(s)

How are star spectra used to help us find stars that are more likely to harbor planets with life?



Students consider the physical characteristics of stars that make stars more likely to support planets with complex life. Students recall or are introduced to star spectra as a tool for remote measurements of star properties.


Students work in a team to sort 14 star spectra and create a classification system for the stars. Teams are paired to create a consensus classification and discuss the communication issues involved in classification processes.


Students study Figures A1–A3 to learn about and apply the canonical OBAFGKM classification system. Using Figures B1–B4 as guides, students learn why light absorption by elements in stars is dependent on temperature.


Students learn more about the history and applications of the OBAFGKM classification system and practice quantitative problem-solving using a blackbody radiation equation.


After reading a NASA article on OBAFGKM star types that are more likely to support planets with life, students use the 4-2-1 strategy with a timed writing or Cognitive Comic activity to identify the most important information in the article and summarize the lesson.


  • How to Classify Stars with Spectra (attached; one per student)

  • How to Classify Stars with Spectra (Sample Responses and Teacher Clarifications) (attached)

  • Data Handout (optional, attached; one per student)

  • Goldilocks Stars Are Best Places to Look for Life article (linked below; one per student)

  • Lesson Slides (attached)

  • Student notebook (optional)

  • Scissors (optional)

  • Glue (optional)

  • Pencil or pen


Use the attached Lesson Slides to guide students through the lesson. Keep in mind that you can edit, add, or omit slides to suit the class’s needs.

Using slides 2-4, introduce the lesson title, essential question, and lesson objectives.

Distribute the attached How to Classify Stars with Spectra packet to students. Consider distributing the packet via Google Docs or email, as it contains a few linked resources for students’ use. Working on their own or with Elbow Partners, have students try to answer the three speculative questions on page 1 of the packet. (You may also make this activity a Bell Ringer.) Have students record their work on the handout or in a physical or online notebook.

Next, move to slide 5, and engage students in a whole-class discussion about what physical characteristics make stars more likely to harbor an orbiting planet with complex life on it, and how we might look for those stars. Move to slide 6, and introduce students to star spectra, spectral absorption lines, and what they can teach us about stars. Your explanation should take into account students’ prior knowledge, gauging it via class discussion where needed. Initiate a review of previous knowledge about light, spectra, and absorption lines, or introduce the fundamental information needed for the Explore phase.


Move to slide 7. Referencing the Explore section (page 2) of the How to Classify Stars with Spectra handout, invite students to look for similarities and differences in 14 star spectra and sort them into groups. Connect students’ thinking to the Engage activity by pointing out that sorting processes like these led to the identification of star groups that are more likely to harbor life.

Use the three spectra shown on the left side of slide 7 to guide students in practicing. Do so by asking students to discuss the similarities and differences they see in these spectra, then to identify the most distinctive spectrum. Most students will conclude that star fiber 2 (at the top-right corner) and star fiber 157 (at the bottom-right corner) are most similar to each other and that star fiber 346 is the most distinctive. Ask students to use the table on their handout (pictured on the right side of the slide) to report their own classification results.

Group students into teams of 2–3 (larger groups are not recommended).

Have students use the method of your choosing—the SDSS Database or Data Handout—to review data. See the instructions below for more information on each.

Have students begin Part A (comparing star spectra to classify 14 stars). Circulate the room as students work to help them interpret data or iron out differences with teammates. Allow 10–30 minutes for students to compare, sort, and classify. How much time is required varies depending on whether students are using the SDSS Database or the Data Handout.

Part B might begin immediately following Part A or at the start of the next class period.

Once students have finished Part A, pair teams to work through Part B (page 4 of the handout). Move to slide 10. Remind students to record written notes of their discussion. Let groups know that their consensus schemes can be reported in a table (like those used in Part A), but also encourage groups to create alternative and better ways to present their classification scheme. For example, a decision tree is one visually strong method of presenting a classification scheme.

Once students finish their discussions, bring the class together for a short discussion of the questions posed in Part B, including when and how scientists should cooperate. This discussion should serve as a chance to evaluate students’ learning so far, as well as a chance to emphasize general points about scientific classification and collaboration.

If needed, guide the discussion toward the idea that independence is needed for the development of new ideas. Additionally, cooperation is needed for synergistic breakthroughs and to confirm (or refute) ideas from another group.

Use this discussion to segue to the classification scheme explored in the Explain activity. In brief, the OBAFGKM classification used in the Explain phase is a middle step in classification systems. It was built by substantially modifying (and improving the usability) of an alphabetical scheme developed by Draper. It was subsequently elaborated to include more complexity and some new data as the Morgan-Keenan scheme. For a fuller explanation of this history, see the COSMOS page on the Harvard Spectral Classification.

A key point is that classification is somewhat arbitrary and depends on the data available and used.


Show students slide 11 to introduce the spectral lines (Table A1) and classification labels (Table A2) used in the OBAFGKM system.

Use slide 12 to clarify the goals of the Explain questions and orient the students to Figure A3: a star spectrum the students will classify using the OBAFGKM system.

Ask students to work with the figures to answer the questions individually or in small groups. Circulate to help students and encourage them to make their own hypotheses. Questions can be assigned as a homework assignment, though students may get stuck and struggle to complete the set without the support of peers or a teacher.

After students have worked through the questions, facilitate a whole-class discussion to review understanding. To identify areas of confusion before or after the discussion, use the Muddiest Point strategy detailed on slide 13.

Use slide 14 to support a discussion of questions A1–A5 in the packet. Use the questions and discussion to introduce students to the OBAFGKM system, referring to questions Tables A1 and A2 for questions about the Sun. Use slides 15–17 to support a discussion of questions A6–A11. This prompts students to characterize the star in Figure A3.

  • Use slide 15 to work out the assignment of the spectra in Figure A3 step by step.

  • Use slide 16 as a reference.

  • Use slide 17 to facilitate a class vote on the assignment.